Flight Asia:
The reason they can clear immigration etc without work permits is they dont live in Australia.
As someone has mentioned already, Qantas has had overseas based cabin crew operating in and out of AUS for over 20 years now.
Take the QF AKL based Longhaul cabin crew, they pax or operate to Australia, do their trip 10 odd day trip and then pax or operate home again. They live and are paid in NZ employed on a local contract, but work to CASA rules as they are operating on VH registered aircraft.
Thai crew used to do the same, Japan crew still do (but mostly operate NRT-SYD-NRT).
It wouldn't be any different for SIN/ BKK based Jetstar crew, they live and are paid in their countries, operate to Australia, and then home via whatever routes their trips dictate.
I don't agree with it, but it would be no different for Jetstar than for Qantas.