It is highly complicated and the AUS crews are under NO obligation to turn down work to keep the flights going (except maybe morally), it would be entirely up to NZALPA and the Virgin NZ pilots to stop them.
It could get tricky for the Oz guys anyways as under what grounds can you legally refuse to go to work? They are already doing NZ branded flying (DPS/POM/NAN/HIR) so what's a few more sectors?
Unless there is a NZ court order stopping them I don't see how an Australian employed pilot could legally refuse to fly NZ and Pacific Sectors allocated to them.