hi; I'm a skyscanner fan, but plenty of other options for checking several airlines; guess that with a family you may not be keen on stopover/s and changing airlines?
one point re one way tickets - from the Air NZ website
People travelling to New Zealand on a one-way ticket must be either:
a New Zealand passport holder
an Australian passport holder
a permanent resident of Australia holding a valid Australian Resident Return Visa allowing an indefinite stay
other foreign national holding a valid New Zealand Returning Resident's Visa
All other visitors to New Zealand must possess a return or onward ticket to a country to which you have right of entry.
see also UK FCO site