Contact AirNZ and ask them to quote you a one way fare. I was in the same situation booking multileg trips intending to return from JFK to AKL (side note: the booking system didn't allow me to book the sequence of journeys I required). The AirNZ US website quoted in USD was nearly triple the price of the equivalent journey as part of an AKL-JFK rtn ticket on a dummy search. I emailed AirNZ and they quoted me the same oneway fare in NZD and bookable directly over the phone.