I was going to post something very similar.The State declarations of emergency apply to that states affairs and won't be affected by any high court decision.
The cases are about the constitution and If the borders can be closed.The letter of the law is no but exceptions can be made in certain situations and that can include a pandemic.
If any Emergency declaration applied to the whole country it would be the one declared by the Commonwealth government and we have heard from the CMO that closure of state borders weren't necessary.
The WA Emergency Management Act grants many and varied powers to the State Emergency Coordinator, who can then appoint authorised officers to undertake any of the permitted actions that the act enables. Curiously, this person is actually the Police Commissioner. So the Police seem to have defacto & complete authority through the act as a result.
I have read parts of the act and my very amateur take away is that it enables the state to manage emergencies as they arise including access & egress from declared areas. But how can the whole state of WA be declared under emergency when there is no given emergency within the state? Similarly I would say the act does not provide the state with the powers to control entry to or from the state - just areas within it. Controlling the borders in the way they are doing appears to be infringing on the free movement provision within the constitution.
Section 117 of the Constitution states
Therefore I personally believe that asking Clive Palmer, an otherwise law abiding subject of the Queen for his intended movements while in the state to be against the constitution, given that others (including those within WA) have no obligation to provide such information. This is not to say that the WA government couldn't find ways to implement their desired policy in other ways - but it is incumbent on all governments to abide the constitution while achieving their policy goals. If the current method is found to be against the constitution, then they need to find a way that complies with it.A subject of the Queen, resident in any State, shall not be subject in any other State to any disability or discrimination which would not be equally applicable to him if he were a subject of the Queen resident in such other State.