David Flint in his article, “Dictatorial Travel Ban Unlawful,” published in the Spectator Australia, August 29, 2020, puts the case, as the title indicates, that the federal government travel bans are illegal.
  https://www.spectator.com.au/2020/08/dictatorial-travel-ban-unlawful/

“Because the National Cabinet has no place in the Constitution, its decisions are affected by ministers making subordinate legislation, often without even viceregal scrutiny. Exercising a power under the Biosecurity Act ‘to prevent or control the spread’ of the virus ‘to another country’, the Health Minister on 25 March decreed that an Australian citizen ‘must not leave Australian territory as a passenger’ without satisfying some faceless bureaucrat that he or she has a ‘compelling reason’ to leave. The reasons are narrowed down on the official website. Officials and business people apart, ‘compelling reasons’ are limited to medical treatment not available in Australia or on compassionate or humanitarian grounds. According to reports, these take time and are rarely granted. This almost blanket ban on travelling is beyond the narrow power parliament gave the minister to stop the spread of the virus to another country. This is a funnel the size of which constrains the minister, whatever the National Cabinet wants, to make subordinate legislation of no greater width. He could have quite properly required that people leaving be tested. But like a Stalinist state, you can’t leave without the agreement of some faceless bureaucrat. This tyrannical requirement could be as much an actionable misfeasance in public office as was the Gillard government’s unnecessary and legally unjustifiable total ban on the export of live cattle to Indonesia. This is but one example of ministers wielding powers unnecessarily and capriciously restricting Australians in the exercise of their fundamental rights, including the common law rights to carry on a business, to work and to move around freely. Power has gone to the politicians’ heads.”

    I have covered the issue of the legality/illegality of the Covid-19 restrictions before, with readers seeking a legal method of challenging all of this. Doesn’t our constitution defend our rights? The common law? Well, yes, and no. The Australian constitution, unlike the American one, is thin on rights, being an enabling document to get the states together in a federation, and make commerce flow.  The thought was that rights would be protected by the courts, which proved to be totally incorrect, as the High Court in the beginning moved to support centralism, and diminish state power in decision after decision. Thus, I do not see any real way of challenging in total the present medical totalitarian legislation.  Particular applications could be challenged though as ultra vires, exceeding the power given in the Act, which is arguably what has occurred in Melbourne, as numerous legal authorities have affirmed. Yet, the politicians if caught out, can always go back to the drafters, and plug the holes, giving them the power that they want. 

     There is thus a major problem in our legal system in how to control the power of the state. Conservatives who see the constitution was perfect do not like this mode of thought, and I feel sorry for them, but the case of Melbourne has proved me right. And, even if there was a method of attack, lawyers, who are so eager to defend refugees’ rights, have not charged into battle to defend the rights of the ordinary Australian.