As much as it pains me to say it, I personally can't see the High Court striking down the provisions of the Biosecurity Act 2015 (Cth) and ministerial determinations that brought all of this about in a constitutional challenge. There are no express constitutional rights associated with citizenship in the Australian Constitution (the word "citizen" is not even used in the Constitution, instead "subjects of the Queen" appears when necessary to refer to Australian persons), and the High Court nowadays is not prone to find implied constitutional rights as it did in the 1990s in Lange and Nationwide News.
The Australian Citizenship Act 2007 (Cth) confers no substantive rights of citizenship. Any benefit of citizenship is conferred by individual statute, for example, the ability of citizens to vote is found in the Commonwealth Electoral Act 1918 (Cth), the ability of citizens to obtain a passport is conferred by the Australian Passports Act 2005 (Cth) and the ability of Australians to leave and re-enter Australia is a negative right arising from the absence of any prohibition on exit and re-entry, or the requirement for permission to depart and return, in the Migration Act 1958 (Cth) (and that negative right can be trumped by another law in accordance with the general principle that all law is subject to other law, which is the case with the ministerial determinations made under the Biosecurity Act that impede the ability of Australians to leave and re-enter Australia).
The international law principle of "right of return" (which appears in the Universal Declaration of Human Rights (to which Australia is a signatory) and the International Covenant on Civil and Political Rights (to which Australia is a signatory, and has ratified it, but has not adopted it into domestic law)) appears to have no force in Australia. I'm not aware of any legal scholars or lawyers who dispute it is within the power of the Parliament to prevent citizens from leaving and returning to Australia.
If there's any basis for a successful challenge, it could potentially lie in challenging executive overreach. The bans are merely executive fiat, made pursuant to powers under the Biosecurity Act. As a general principle, English and Australian common law is hostile to the use of executive power as a substitute for a proper exercise of parliamentary power. The criminalising of the entry into Australia by a citizen who was in India within the last 14 days (which has happened with the making of the Biosecurity (Human Biosecurity Emergency) (Human Coronavirus with Pandemic Potential) (Emergency Requirements—High Risk Country Travel Pause) Determination 2021) is a big deal so far as a traditional and legally conservative view of the proper use of executive power is concerned. Criminalising things and invoking the penal power of the state is traditionally not something done lightly and then only after full debate in a representative parliament. I think this ministerial determination is indefensible, but the High Court has become a bastion of protecting and expanding Commonwealth executive (and parliamentary) power*, so I think there is little hope for any challenge in the High Court.
* Which isn't really the High Court's fault, but simply reflects Australia's constitutional arrangements. Our Constitution is a bare bones framework document for governance of the Commonwealth, and that's about it. The High Court makes no secret of its very textualist and black letter law approach to constitutional interpretation. Using that approach, there's not much in the text of the Constitution for the High Court to work with.
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