Willie Nelson.
It has nothing to do with trade agreements. It's to do with the Australian EBA. If Jetstar could, they'd employ everybody under the NZ agreement and roster them through Australia. They'd do this in a heartbeat because it's cheaper! The only thing stopping them is the union(s) and the EBA.
If they operate a domestic Australian route and aircraft, they need to be on the Australian seniority list and paid the Australian rate under the eba.