As above, if Auckland is not the final NZ destination flying USA to SYD-MEL-BNE and then a flight to a NZ airport can be better than transiting AKL. (far more international airports in NZ other than AKL). Open jaw is a good option.
Most here would recommend the South Island (~mainland) over the North Island.
Rental car availability in late Dec/Jan can be an issue. Book early.
And
NZeTA required from 1 October 2019 to visit or transit New Zealand