Luke,
I have just been down this road and went the way of doing the CASA ATPL conversion. From my experience I ended up spending about the same as I would have if I had just hired the Air NZ sim. You will have to gain your CASA medical then apply for a Certificate of Validation to enable you to do the training in Oz. This is a very frustrating process and took me months to sort. Then you have to sit the CPL LAW exam which gives you the right to sit the ATPL LAW exam which in turn allows you to sit the IREX (instrument exam). These took me 2 weeks pretty much full time self study. You can't book the subsequent exams until you pass the previous, so you have to do all this once you are in OZ. With the exams passed you have to pass the IR flight test which took me two sessions in an FNPT II followed by one flight in a Duchess and the test. You then submit all of this to CASA and wait up to 6 weeks for you licence. I spent 3 weeks full time in Perth to get all of this done and it was very hard work. All in it cost me about $5,000 AUD, AIR NZ quoted me $5,500 NZD for a practice LPC followed by an LST. So cost wise about the same, I know people who have done the NZ conversion and it took them a week in total. If I had it all to do again I think I would have just hired the A320 sim and be done with it. Hope this helps.