Pursuant to the Airports Act 1996, airport‑operator companies are subject to a 49% limit on ownership by foreign persons. By 'foreign persons' is notably understood foreign citizens not ordinarily resident in Australia, or companies where foreign citizens not ordinarily resident in Australia hold  a particular type of stake in the company of not less than 15% individually or not less than 40% in aggregate.
"40 Meaning of unacceptable foreign‑ownership situation
 For the purposes of this Act, an unacceptable foreign‑ownership situation exists in relation to an airport‑operator company if there is a group of foreign persons who hold, in total, a particular type of stake in the company of more than 49%.
[...]
42 Compliance by airport‑operator company
 (1) An airport‑operator company must take all reasonable steps to ensure that an unacceptable foreign‑ownership situation does not exist in relation to the company. [...]
Schedule—Ownership definitions
2 Definitions
 (1) In Part 3 and this Schedule, unless the contrary intention appears: [...]
foreign person means:
 (a) a foreign citizen not ordinarily resident in Australia; or
 (b) a foreign government body; or
 (c) a company where:
 (i) a foreign citizen not ordinarily resident in Australia; or
 (ii) a foreign government body; or
 (iii) a foreign company;
 holds a particular type of stake in the company of not less than 15%; or
 (d) a company where a group of 2 or more persons, each of whom is:
 (i) a foreign citizen not ordinarily resident in Australia; or
 (ii) a foreign government body; or
 (iii) a foreign company;
 holds, in total, a particular type of stake in the company of not less than 40%; or
 (e) the trustee of a trust estate in which:
 (i) a foreign citizen not ordinarily resident in Australia; or
 (ii) a foreign government body; or
 (iii) a foreign company;
 holds a substantial interest; or
 (f) the trustee of a trust estate in which 2 or more persons, each of whom is:
 (i) a foreign citizen not ordinarily resident in Australia; or
 (ii) a foreign government body; or
 (iii) a foreign company;
 hold an aggregate substantial interest. [...]" (Airports Act 1996 (Act No. 42 of 1996), last amended by National Emergency Declaration (Consequential Amendments) Act 2020 on 16 December 2020, Sections 40, 42 and Schedule, Section 2)
Airports Act 1996 (Act No. 42 of 1996), last amended by National Emergency Declaration (Consequential Amendments) Act 2020 on 16 December 2020, Sections 40, 42 and Schedule, Section 2
[<a href="https://www.legislation.gov.au/Details/C2021C00019">Act</a>]