If you intend forging a career in GA in Australia then do your IFR training in Australia. Yes, you can do it in NZ and get it put on your CASA license at first issue. And it might be a bit cheaper in NZ, I don't know. But chances are you might have to do some re-training in Australia at some stage anyway. So will it work out cheaper?
If you're looking for a VFR job in Australia that doesn't require an IFR rating then the cheapest option would be to not get an IFR rating yet. Get your foot in the door in GA in Australia, then do an IR rating in Australia when you're ready for a multi IFR job there. If you go across with a fresh NZ IFR rating thinking you're ready to fly IFR in Australia then you'll get an unpleasant surprise. Quite a few little differences in the Australian instrument rating exam syllabus to the NZ syllabus.