As far as I understand it, NSW ACT and Vic borders have not been closed.
WA NT SA QLD & Tas borders have been closed.
That means, for instance... I can drive from Sadelaide into Vic or NSW no problemo.. but I can't get back into SA without self isolating.
I know it doesn't make much sense to some... but if a State (or Territory) hasn't closed it's borders and a neighbouring State has... The Police in the closed State can only stop you from coming in... they have no durisdiction to stop you leaving (unless you've committed a crime whilst visiting, of course
)
There may be rules in NSW & Vic around travelling about for no good reason etc though... so you might just want to check you're not falling foul of anything like that !??
D