We have a constitution, too.
And our states are responsible for their own decisions. Hence the closing of some states to other states. No-one told them to do that. It is their own directive.
However, what we are doing right .... all state leaders have been consulting together in National Cabinet (phone call meetings/online meetings/physical meetings when it is safe) where they plan strategy, work out their differences, and get on the same page.
This provides good cohesive leadership for us all to follow. It helps us reach a mutual goal that is good for the country.
Please remember that Australia is geographically as big as the US. People from different areas think in different ways, approach things in different ways, BUT cohesive leadership from all states are pulling the nation's approach together.
The Australian Constitution:
The Australian Constitution – Parliament of Australia