Hi cnc2002,
I did a quick search in the middle of march on AirNZ.co.nz, and I'm finding a whole bunch of fares available between $59 and $99 each way. These are Air NZ's "GrabASeat" seat-only fares (limited flexibility and no luggage allowance). For $10 extra you get a checked bag, and maximum change fee on these fares is $50 I believe (up till day before departure).
Where are you searching, cause I don't see why these fares should no show for you?