The bottom line, is that REX can employ whomever they want.
Ah, no, in fact they can't. Rex would first need to be able to prove to the appropriate immigration department that no Australians were available to do this job, something I'm surprised to hear exists. Once that is done they are free to sponsor the individuals under appropriate visas. Frankly I'm surprised this one is sliding past the relevant pilot union(s), so it must simply go to prove there truly is a pilot shortage in Australia.