If the segments are book with operating airline flight number (i.e. AC, NZ, NZ. AC), then this should be possible to do by having the agent specify the FQTV for AC to be AC and for NZ to be UA. It may even be possible by segment. If it's booked as codeshares (i.e. NZ*AC, NZ, NZ, NZ*AC). then it's much harder to do.
Whether you will get a competent agent from NZ who knows how to do this is a different matter.