At least until they comply with Australian workplace law. They may be permitted to operate domestically within Australia under CASA regs but I suspect the FWC would take a very dim view of them operating domestically whilst on different terms and conditions to that which applies to Australian crew.
Of course were they to be subject to the Qantas SH EA then there’d be no restrictions on them operating domestically. I’m sure the JC crew would love the pay rise and better conditions. Of course there’d be knock on effects that they may not like also.