Putting regional Victoria into lockdown (unnecessarily) triggered a message to other states that Vic considered it a hotspot / area of concern and therefore restrictions were mirrored interstate.
Sometimes other states jump the gun and declare on their own, but for the last few months if a state self declares a hotspot as Queensland, WA and now Victoria have done - the other states follow suit.
History has shown that both WA and Queensland wait between 14-28 days of a declaration being lifted to allow green travel. Other states generally faster, but it can vary, NT typically the fastest. This is why you need to make declarations/restrictions targeted to actual areas of concern and not statewide.
Had Vic not lumped Regional Vic in with Metropolitan Melbourne, you can bet that the interstate restrictions would have not applied to Regional Victoria, only Melbourne metro. This is what happened with the Brisbane and Perth declarations, other states didn't put restrictions on travelers from Broome or Cairns for example.
The more nuanced approach was starting to gain momentum, until it regressed with this latest statewide lockdown.