Got an email from NZ CAA that doing a checkride in Australia will cost a candidate at least more than $5K to $10K to cover NZ CAA inspector's fees.
I need to do the conversion sometimes later this year as well and it seems the cheapest way is to get it done within the Air New Zealand Sim Center however you must be typed in an airplane that Air NZ sim center has its sims for. Thus B777, B737CL, A320 etc only.
NZ CAA also said FAA ATP and Type Rating Circ Apch - VMC Only aren't need to be removed prior to the conversion because you will demonstrate Circ Apch in IMC during the chrckride.
In addition to that, you must have N-reg 500 hrs PIC or 1000 hrs SIC in multi-crew airplanes (that requires 2 pilots by FAR Part 25) to do a simple Air Law + checkride instead of full ATPL theories.
This is all I know.
Cheers.