This is going to be a very interesting test case, and many top lawyers are studying this at present. The premier was the one that made the call on this, not the police. It has now come out, that it was done to make life easier for the police and has no actual bearing on the current situation.
https://www.dailymail.co.uk/news/article-8716275/Dictator-Dan-Andrews-admits-Melbourne-night-curfew-easier-cops-enforce-lockdown.html read this first.
