I checked the official guidelines yesterday and the Cth govt requires all cruise passengers to do a RAT or PCR before boarding. There’s also a raft of measures cruise ships have to do in order to be allowed to sail.
The rules against isolation don’t seem to apply to travelling on a cruise unfortunately

If you have covid you can’t board.
The Cth’s reasoning behind the rules seems logical when you read them… 3000 pax compared to 300 on a plane, lots of activity and talking, lots of sustained close contact with others (days rather than hours on a plane).
I’m sure there will be some who fake the test, but ultimately it’s a Cth, not cruise matter. And I guess there are penalties for a false declaration.