Under the Foreign Acquisitions and Takeovers Act 1975, certain 'significant actions' can be prohibited by the Treasury if considered to be contrary to the 'national interest'. These include actions to acquire interests in securities, assets or Australian land, and actions taken in relation to entities (being corporations and unit trusts) and businesses, that have a connection to Australia. In general, a change in control and meeting a monetary threshold test are required for an action to be significant. The national interest, and what would be contrary to it, is not defined in the Act or the Foreign Acquisitions and Takeovers Regulation 2015. Assessments are made on a case-by-case basis. However, a Foreign Investment Policy document states that the Government typically considers the following factors when assessing foreign investment proposals: national security; competition; other Australian government policies, including tax; impact on the economy and the community and the character of the investor. 
Additionally, any 'notifiable action' must be notified to the Treasury before the action can be taken. A notifiable action is a proposed action by a foreign person to acquire a direct interest in an Australian entity or Australian business that is an agribusiness, to acquire a substantial interest (generally at least 20%) in an Australian entity or to acquire an interest in Australian land. Generally, the action is notifiable when a monetary threshold test is met. The monetary screening thresholds concern either the amount paid for an interest or the value of an entity or assets. The relevant threshold depends on the type of investor and type of action taken. Generally, a higher threshold applies for investors from certain countries with which Australia has free trade agreements when these investors invest in non-sensitive businesses. Sensitive businesses, as defined in the Regulation, include transport, media and telecommunications businesses, among others. From 1 January 2021, the temporary $0 screening thresholds, introduced in 2020 in response to the COVID-19 pandemic, no longer apply. Instead, the prior tiered threshold regime is reinstated. For a table of all the applicable thresholds in 2021, see Treasury, "Australia's Foreign Investment Policy" (1 January 2021), Annex 1, pp. 15-16.
Certain acquisitions are exempted from reviews by virtue of the 2015 Regulation. Generally, acquisitions by foreign non-government acquirers of an Australian business carried on by, or an acquisition of an interest in Australian land from the Commonwealth, a State, a Territory or a local governing body of Australia or an entity wholly owned by the Commonwealth, a State, a Territory or a local governing body, are exempted from reviews. However, acquisitions of an interest in Australian land if the interest is, or includes, certain infrastructure assets, are excluded from the exemption. These infrastructure assets include 'public infrastructure', which is defined in the Regulation as including also electricity generation, transmission, distribution and supply systems and facilities.
From 1 January 2021, amendments to the Act and Regulation introduced, among other changes, the possibility to screen certain investments under a narrower 'national security test' when these investments do not meet the monetary and control thresholds of the already existing mechanisms but nonetheless pose national security concerns. For the purposes of the national security test, two new categories of actions, 'notifiable national security actions' and 'revieweable national security actions', and the concept of 'national security business', were introduced in the Act and Regulation. Examples of national security businesses include responsible entities and direct interest holders of critical infrastructure assets, as defined in the Security of Critical Infrastructure Act 2018, and carriers or nominated carriage service providers to which the Telecommunications Act 1997 applies.
The Security Legislation Amendment (Critical Infrastructure) Act 2021, in force as of December 2021, expanded the scope of the Security of Critical Infrastructure Act 2018 to include critical infrastructure entities in a wider range of sectors including: communications; financial services and markets; data storage or processing; defence industry; higher education and research; energy; food and grocery; health care and medical; space technology; transport; and water and sewerage. 
The Security Legislation Amendment (Critical Infrastructure) Act 2022, in force as of April 2022, revisited some of the critical infrastructure assets definitions and introduce enhanced cybersecurity obligations for highly critical assets.
"3 Object
The object of this Act [...] including by: [...]
(c) requiring responsible entities for critical infrastructure assets to identify and manage risks relating to those assets; and
(d) imposing enhanced cyber security obligations on relevant entities for systems of national significance in order to improve their preparedness for, and ability to respond to, cyber security incidents; and
[...]
5 Definitions
In this act [...]
critical component of a critical infrastructure asset, means a part of the asset, where absence of, damage to, or compromise of, the part of the asset:
(a)  would prevent the proper function of the asset; or
(b)  could cause significant damage to the asset;
as assessed by the responsible entity for the asset.
[...]
[text not reproduced: new and amended the definitions of several critical infrastructure assets  - see legislation link for the full information]
[...]
8G  Meaning of relevant impact [...]
(3)  Each of the following is a relevant impact of a cyber security incident on a system of national significance:
(a)  the impact (whether direct or indirect) of the incident on the availability of the system;
(b)  the impact (whether direct or indirect) of the incident on the integrity of the system;
(c)  the impact (whether direct or indirect) of the incident on the reliability of the system;
(d)  the impact (whether direct or indirect) of the incident on the confidentiality of:
(i)  information about the system; or
(ii)  if information is stored in the system—the information; or
(iii)  if the system is computer data—the computer data.[...]" (Security of Critical Infraestructure Act 2018, (No. 29, 2018), last amended by by Security Legislation Amendment (Critical Infraestructure Protection) Act 2022 on 2 April 2022, Sections 3, 4, 5 and 8G)
-------------------------
"4  Dictionary of definitions
In this Act:[ ...]
foreign person means:
(a)  an individual not ordinarily resident in Australia; or
(b)  a corporation in which an individual not ordinarily resident in Australia, a foreign corporation or a foreign government holds a substantial interest; or
(c)  a corporation in which 2 or more persons, each of whom is an individual not ordinarily resident in Australia, a foreign corporation or a foreign government, hold an aggregate substantial interest; or
(d)  the trustee of a trust in which an individual not ordinarily resident in Australia, a foreign corporation or a foreign government holds a substantial interest; or
(e)  the trustee of a trust in which 2 or more persons, each of whom is an individual not ordinarily resident in Australia, a foreign corporation or a foreign government, hold an aggregate substantial interest; or
(f)  a foreign government; or
(g)  any other person, or any other person that meets the conditions, prescribed by the regulations. [...]
substantial interest: a person holds a substantial interest in an entity, trust or unincorporated limited partnership if:
(a)  for an entity or unincorporated limited partnership—the person holds an interest of at least 20% in the entity or partnership; or
(b)  for a trust (including a unit trust)—the person, together with any one or more associates, holds a beneficial interest in at least 20% of the income or property of the trust.
[...]
40  Meaning of significant action—entities
(1)  An action is a significant action if the conditions in this section are met.
First condition—kinds of action
(2)  The first condition is that the action is any of the following:
(a)  to acquire a direct interest in an Australian entity that is an agribusiness;
(b)  to acquire interests in securities in an entity;
(c)  to issue securities in an entity;[ ...]
Second condition—threshold
(3)  The second condition is that the threshold test is met in relation to the entity (see Subdivision B of Division 4).
Third condition—kinds of entities covered
(4)  The third condition (except if paragraph (2)(a) applies) is that the entity is the following:
(a)  for an acquisition of interests in shares or an issue of shares:
(i)  a corporation that is a relevant entity that carries on an Australian business, whether alone or together with one or more other persons; or
(ii)  the holding entity of such a corporation;
(b)  for any other action relating to a corporation:
(i)  an Australian corporation that carries on an Australian business, whether alone or together with one or more other persons; or
(ii)  the holding entity (other than a foreign corporation) of such a corporation;
(c)  for an action relating to a unit trust:
(i)  an Australian unit trust; or
(ii)  a holding entity of an Australian unit trust.
Last condition—action taken by a foreign person
(5)  If paragraph (2)(a) applies, the last condition is that the action is or is to be taken by a foreign person.
Last condition—action results in change in control
(6)  If any of paragraphs (2)(b) to (e) apply, the last condition is that there would be or has been a change in control of the entity as a result of the action (see Subdivision C of Division 4).
(7)  However, the last condition in subsection (6) is not required to be met if:
(a)  paragraph (2)(b) applies and the action is or is to be taken by a foreign person who controls the entity (whether alone or together with one or more associates) immediately before the action is or is to be taken; or
(b)  paragraph (2)(c) applies and a foreign person controls the entity (whether alone or together with one or more associates) immediately before the action is or is to be taken.
41  Meaning of significant action—businesses
(1)  An action is a significant action if the conditions in this section are met.
First condition—kinds of action
(2)  The first condition is that the action is any of the following:
(a)  to acquire a direct interest in an Australian business that is an agribusiness;
(b)  to acquire interests in assets of an Australian business;[...]
Second condition—threshold
(3)  The second condition is that the threshold test is met in relation to the Australian business (see Subdivision B of Division 4).
Third condition—action taken by a foreign person
(4)  If paragraph (2)(a) applies, the third condition is that the action is or is to be taken by a foreign person.
Third condition—action results in change in control
(5)  If paragraph (2)(b) or (c) applies, the third condition is that there would be or has been a change in control of the business as a result of the action (see Subdivision C of Division 4).
(6)  However, the third condition in subsection (5) is not required to be met if:
(a)  paragraph (2)(b) applies; and
(b)  the action is or is to be taken by a foreign person who controls the business (whether alone or together with one or more associates) immediately before the action is or is to be taken.
[...]
47  Meaning of notifiable action—general
(1)  An action is a notifiable action if the conditions in this section are met.
First condition—kinds of action
(2)  The first condition is that the action is any of the following:
(a)  to acquire a direct interest in:
(i)  an Australian entity that is an agribusiness; or
(ii)  an Australian business that is an agribusiness;
(b)  to acquire a substantial interest in an Australian entity;
(c)  to acquire an interest in Australian land.
Second condition—threshold
(3)  The second condition is that the threshold test is met in relation to the entity, business or land (see Subdivision B of Division 4).
(4)  If subparagraph (2)(a)(i) or paragraph (b) applies, the third condition is that the entity is:
(a)  an Australian corporation that carries on an Australian business, whether alone or together with one or more other persons; or
(b)  an Australian unit trust; or
(c)  an Australian entity that is the holding entity of an entity mentioned in paragraph (a) or (b).
Last condition—action taken by a foreign person
(5)  The last condition (whether paragraph (2)(a), (b) or (c) applies) is that the action is or is to be taken by a foreign person.
[...]
51  The threshold test for entities and businesses
The threshold test is met in relation to an entity or business for a significant action mentioned in column 1 of the following table if the value mentioned in column 2 of the table is more than the value prescribed by the regulations.
[see legal source for table]
[...]
55B  Meaning of notifiable national security action
(1)  An action is a notifiable national security action if the action is taken, or proposed to be taken, by a foreign person and the action is any of the following:
(a)  to start a national security business;
(b)  to acquire a direct interest in a national security business;
(c)  to acquire a direct interest in an entity that carries on a national security business;
(d)  to acquire an interest in Australian land that, at the time of acquisition, is national security land;
(e)  to acquire a legal or equitable interest in an exploration tenement in respect of Australian land that, at the time of acquisition, is national security land.
(2)  For the purposes of paragraph (1)(e), a person acquires a legal or equitable interest in an exploration tenement even if:
(a)  the person has previously acquired an interest in an exploration tenement; or
(b)  the interest is an increase in the amount of an existing interest of the person in an exploration tenement.
(3)  The regulations may provide, in relation to exemption certificates in force under regulations made for the purposes of section 63, that an action of a specified kind is not a notifiable national security action.
[...]
55D  Meaning of reviewable national security action—entities
Meanings
(1)  An action is a reviewable national security action if:
(a)  the action is taken, or proposed to be taken, by a person and the action is to acquire an interest of any percentage in an entity; and
(b)  as a result of the action or proposed action:
(i)  a foreign person acquires, or will acquire, a direct interest in the entity and that acquisition, or proposed acquisition, is not a significant action, notifiable action or notifiable national security action; or
(ii)  a foreign person will be in a position, or more of a position, to influence or participate in the central management and control of the entity; or
(iii)  a foreign person will be in a position, or more of a position, to influence, participate in or determine the policy of the entity; and
(c)  the action is or was not otherwise a significant action, a notifiable action or a notifiable national security action.
(2)  An action is also a reviewable national security action if:
(a)  the action is taken, or proposed to be taken, by a person and the action is:
(i)  to issue securities in an entity; or
(ii)  to enter an agreement relating to the affairs of an entity and under which one or more senior officers of the entity will be under an obligation to act in accordance with the directions, instructions or wishes of a foreign person who holds a direct interest in the entity; or
(iii)  to alter a constituent document of an entity as a result of which one or more senior officers of the entity will be under an obligation to act in accordance with the directions, instructions or wishes of a foreign person who holds a direct interest in the entity; and
(b)  in the case to which subparagraph (a)(i) applies—as a result of the action or proposed action:
(i)  a foreign person acquires, or will acquire, a direct interest in the entity and that acquisition, or proposed acquisition, is not a significant action, notifiable action or notifiable national security action; or
(ii)  a foreign person will be in a position, or more of a position, to influence or participate in the central management and control of the entity; or
(iii)  a foreign person will be in a position, or more of a position, to influence, participate in or determine the policy of the entity; and
(c)  the action is or was not otherwise a significant action, a notifiable action or a notifiable national security action.
Definitions of entity
(3)  For the purposes of subsection (1), the entity must be:
(a)  for an acquisition of interests in shares:
(i)  a corporation that carries on an Australian business, whether alone or together with one or more other persons; or
(ii)  the holding entity of such a corporation; or
(b)  for any other action relating to a corporation:
(i)  an Australian corporation that carries on an Australian business, whether alone or together with one or more other persons; or
(ii)  the holding entity (other than a foreign corporation) of such a corporation; or
(c)  for an action relating to a unit trust:
(i)  an Australian unit trust; or
(ii)  a holding entity of an Australian unit trust.
(4)  For the purposes of subsection (2), the entity must be:
(a)  for an acquisition of interests in shares or an issue of shares:
(i)  a corporation that is a relevant entity that carries on an Australian business, whether alone or together with one or more other persons; or
(ii)  the holding entity of such a corporation; or
(b)  for any other action relating to a corporation:
(i)  an Australian corporation that carries on an Australian business, whether alone or together with one or more other persons; or
(ii)  the holding entity (other than a foreign corporation) of such a corporation; or
(c)  for an action relating to a unit trust:
(i)  an Australian unit trust; or
(ii)  a holding entity of an Australian unit trust.
55E  Meaning of reviewable national security action—businesses
(1)  An action is a reviewable national security action if:
(a)  the action is taken, or proposed to be taken, by a person and the action is any of the following:
(i)  to acquire an interest of any percentage in an Australian business;
(ii)  to acquire an interest in the assets of an Australian business;
(iii)  to enter or terminate a significant agreement with an Australian business; and
(b)  as a result of the action or proposed action:
(i)  a foreign person acquires, or will acquire, a direct interest in the Australian business and that acquisition, or proposed acquisition, is not a significant action, notifiable action or notifiable national security action; or
(ii)  a foreign person will be in a position, or more of a position, to influence or participate in the central management and control of the Australian business; or
(iii)  a foreign person will be in a position, or more of a position, to influence, participate in or determine the policy of the Australian business; and
(c)  the action is or was not otherwise a significant action, a notifiable action or a notifiable national security action.
(2)  An action is also a reviewable national security action if:
(a)  the action is taken, or proposed to be taken, by a foreign person and the action is to start an Australian business; and
(b)  the action is or was not otherwise a significant action, a notifiable action or a notifiable national security action.
[...]
66A  National security review of actions
Review allowed if action may pose a national security concern
(1)  Subject to subsection (5), the Treasurer may review:
(a)  a reviewable national security action; or
(b)  a significant action that is not a notifiable action or notifiable national security action;
if the Treasurer considers that the action may pose a national security concern.
[...]
67  Order prohibiting proposed significant actions
(1)  The Treasurer may, by notifiable instrument, make an order under subsection (2) if the Treasurer is satisfied that:
(a)  a significant action is proposed to be taken; and
(b)  taking the significant action would be contrary to the national interest.
(1A)  The Treasurer may, by notifiable instrument, make an order under subsection (2) if the Treasurer is satisfied that:
(a)  an action of any of the following kinds is proposed to be taken:
(i)  a notifiable national security action that is not a significant action;
(ii)  an action in respect of which the Treasurer has given a notice under subsection 66A(4);
(iii)  a reviewable national security action notified to the Treasurer; and
(b)  taking the action would be contrary to national security.
(2)  The Treasurer may make an order in accordance with the following table. [...] 
Powers of Treasurer to prohibit proposed actions
Item | If the action is … | the Treasurer may make an order prohibiting …
1 | to acquire a direct interest in an Australian entity that is an agribusiness, or an interest in securities in an entity | the whole or a part of the proposed acquisition.
2 | to issue securities in an entity | the whole or a part of the proposed issue of the securities. [...]
5 | to acquire a direct interest in an Australian business that is an agribusiness, or an interest in assets of an Australian business | the whole or a part of the proposed acquisition. [...]
8 | to acquire an interest in Australian land  | the proposed acquisition.
9 | to take any significant action prescribed by regulations made for the purposes of section 44 | any thing prescribed by regulations made for the purposes of this item.
[...]
81  Compulsory notice of notifiable actions
(1)  A foreign person who proposes to take a notifiable action must give a notice to the Treasurer before taking the action.
(2)  If a person takes an action by entering an agreement, the notice must deal with each notifiable action covered by the agreement.[...]" (Foreign Acquisitions and Takeovers Act 1975 (Act No. 92 of 1975), last amended by Australia’s Foreign Relations (State and Territory Arrangements) (Consequential Amendments) Act 2020 on  1 January 2021, Sections 4, 40, 41, 47, 51, 55B, 55D, 55E, 66A, 67 and 81)
-------------------------
"8AA  Meaning of national security business
(1)  A business is a national security business if:
(a)  the business is carried on wholly or partly in Australia whether or not in anticipation of profit or gain; and
(b)  it is publicly known, or could be known upon the making of reasonable inquiries, that the business is of a kind referred to in subsection (2).
(2)  A business is of a kind referred to in this subsection if:
(a)  the business is a responsible entity (within the meaning of the Security of Critical Infrastructure Act 2018) for an asset; or
(b)  the business is an entity that is a direct interest holder in relation to a critical infrastructure asset (within the meaning of those terms in the Security of Critical Infrastructure Act 2018); or
(c)  the business is a carrier or nominated carriage service provider to which the Telecommunications Act 1997 applies; or
[...]
22  Businesses that are sensitive businesses
(1)  This section prescribes conditions for subsection 26(1) of the Act.
(2)  A business is a sensitive business if:
(a)  the business is carried on wholly or partly in any of the following sectors (including such a business relating to infrastructure for those sectors):
(i)  media; (ii)  telecommunications; (iii)  transport; or
[...]
Division 3—Exemptions for certain actions from being significant and notifiable actions
28  Application of this Division
This Division applies to the provisions of the Act, other than:
(a)  the definition of foreign person in section 4 of the Act; and
(b)  any other provision of the Act to the extent that it relates to that definition.
The provisions of the Act that this Division applies to are the excluded provisions.
[...]
31  Australian businesses carried on by or land acquired from government
(1)  The excluded provisions do not (subject to subsection (2)) apply in relation to an Australian business that is carried on (whether alone or together with one or more other persons) by, or an acquisition of an interest in Australian land from, any of the following persons:
(a)  the Commonwealth, a State, a Territory or a local governing body;
(b)  an entity wholly owned by the Commonwealth, a State, a Territory or a local governing body.
(2)  However, subsection (1) does not apply in relation to: [...]
 (b)  an acquisition of an interest in Australian land if the interest is, or includes, an interest in any of the following infrastructure:
(i)  public infrastructure, except in relation to public roads;
(ii)  infrastructure for existing or proposed roads, existing or proposed railways, or existing or proposed inter‑modal transfer facilities, within the National Land Transport Network (within the meaning of the National Land Transport Act 2014);
(iii)  infrastructure for existing or proposed roads, existing or proposed railways, or existing or proposed inter‑modal transfer facilities, that are designated under a law of a State or Territory as either significant or controlled by the State or Territory;
(iv)  the infrastructure (or part of the infrastructure) of a telecommunications network;
(v)  a nuclear facility; or
(c)  an Australian business that holds an interest in Australian land to which paragraph (b) applies
[...]
41  Exemptions for certain acquisitions [as amended in 2022] […]
(2)  The provisions specified in subsection (1) of this section do not apply in relation to a person’s acquisition of an interest in securities in an entity (including securities in a land entity) if:
(a)  the person’s acquisition of the interest is under, and the interest has not been previously offered for issue under, any of the following:
(i)  a rights issue (within the meaning of the Corporations Act 2001);
(ii)  a rights issue (within the meaning of a law of a foreign country or a part of a foreign country) if the issue is regulated by a law of that foreign country or that part of a foreign country;
(iii)  an issue of securities that is similar to a rights issue (within the meaning of the Corporations Act 2001) and is an issue of a kind covered by an instrument made by ASIC under that Act; or
(b)  all of the following apply:
(i)  the entity is listed for quotation in the official list of a stock exchange in Australia;
(ii)  that listing is the entity’s primary listing in an official list of a stock exchange;
(iii)  the person’s acquisition is under a dividend reinvestment plan, a bonus share plan, a distribution reinvestment plan or a switching facility; or
(c)  all of the following apply:
(i)  immediately before the person’s acquisition, the person holds an interest of a particular percentage (the person’s allowable percentage) in the entity;
(ii)  the person’s acquisition is of a kind whereby additional interests in the entity are only made available for acquisition by each holder of existing interests in the entity in a proportion equal to the percentage interest that such a holder holds in the entity immediately before the acquisition;
(iii)  the portion acquired by the person, and all of the person’s associates, of those additional interests made available to all holders of existing interests in the entity, expressed as a percentage, is no more than the person’s allowable percentage;
(iv)  at the time of the person’s acquisition, the person has no reasonable grounds to believe that the percentage interest that the person will hold in the entity after the last of the acquisitions of those additional interests will exceed the person’s allowable percentage.
[...]
51  Taking action in relation to entities and businesses 
The following table prescribes values for section 51 of the Act (entities and businesses) to the extent that that section relates to items 2 to 5 of the table in that section. 
Threshold value for entities and businesses
Item | For action taken by … |  the value (in millions of dollars) is …
1 | an agreement country or region investor where the action relates to: (a) an entity, or a subsidiary of an entity, that is not carrying on a sensitive business or a trustee of a trust that does not hold assets of a sensitive business; or (b) a business that is not a sensitive business | 1,192             
2 | any foreign person in any other circumstances | 275.[...]" (Foreign Acquisitions and Takeovers Regulation 2015 (SLI 2015 No. 217), last amended by Foreign Investment Reform (Protecting Australia’s National Security) Regulations 2020 on 1 January 2021, Sections 8AA, 22, 28, 31 and 51)
Security of Critical Infraestructure Act 2018, (No. 29, 2018), last amended by by Security Legislation Amendment (Critical Infraestructure Protection) Act 2022 on 2 April 2022, Sections 3, 4 , 5 and 8G.
Foreign Acquisitions and Takeovers Act 1975 (Act No. 92 of 1975), last amended by Australia’s Foreign Relations (State and Territory Arrangements) (Consequential Amendments) Act 2020 on  1 January 2021, Sections 4, 40, 41, 47, 51, 55B, 55D, 55E, 66A, 67 and 81
Foreign Acquisitions and Takeovers Regulation 2015 (SLI 2015 No. 217), amended by Foreign Acquisitions and Takeovers Amendment Regulations 2022 on 31 March 2022
[<a href="https://www.legislation.gov.au/Details/C2022C00160">Act</a>],[<a href="https://www.legislation.gov.au/Details/C2022A00033">Amendment</a>],[<a href="https://www.legislation.gov.au/Details/C2021C00022/Html/Text">Act</a>],[<a href="https://www.legislation.gov.au/Details/F2021C00062">Reg</a>],[<a href="https://www.legislation.gov.au/Details/F2022L00465">Amendment</a>],[<a href="https://foreigninvestment.gov.au/guidance/general">Other</a>],[<a href="https://foreigninvestment.gov.au/investing-in-australia/foreign-investment-framework">Other</a>]