For better or worse, Australia is a federal state and individual states have a wide degree of freedom to impose rules on their populace.

I'm a little surprised we didn't see similar moves in the US, but COVID spread so quickly there that it would have been kind of meaningless. The maritime provinces of Canada, though, had a very similar arrangement at one point, effectively cutting them off from the rest of the country.

All that said, I do think Australia is on a very slippery slope here and the current government is no fan of civil liberties. The restrictions on leaving Australia are particularly indefensible.