This article suggests further legal challenges to the ban could come:
Legal experts are questioning measures forcing travellers to get exemptions to leave the country, and Australia's other responses to the pandemic could increase the chances of a legal challenge.
www.abc.net.au
And those challenges are very unlikely to succeed.
The trend of case outcomes in constitutional cases in the High Court over the last 50 years is to expand federal power, and to find ways to make federal government actions constitutional and lawful. The High Court says that is because that's what the Constitution says. Notwithstanding what those quoted in the article say, against the balance of High Court case law, the federal government is on strong footing with its powers under the Constitution in the following placita of section 51 to keep the exit restrictions in place for as long as it wants:
(ix) quarantine
(xxvii) immigration
and emigration [my emphasis]
(xx_ix) matters incidental to the execution of any power vested by this Constitution in the Parliament
In the context of a public health emergency / global pandemic, the High Court is not going to second-guess parliamentary or executive decisions. I also think section 51(xxvii) provides a stand alone basis for the constitutionality of the exit restrictions in the express words "and emigration". The suggestion that "emigration" means a permanent departure is not likely to be viewed with favour by the High Court, as the meaning of "immigration" has been extended to include temporary entry to Australia. By parity of reasoning, emigration would include temporary departure.
Challenges based on things not incorporated into Australian domestic law such as UN conventions will go absolutely nowhere.
I realise that the exit restrictions are frustrating. I am personally affected by them too. But legal challenges are likely to be a waste of time and money. Australia being a happy-go-lucky, freedom-living place is a myth. The reality is that Australia is highly and tightly (over) regulated and (over) legislated, with federal (which includes the High Court) courts which take an expansive view of, and defer to, federal government power.