How do I use my credit? (as per AirNZ website)
Where: You can use your credit to purchase any Air New Zealand fares, including when another airline is involved. You can also use an existing credit to pay for a flight change, purchasing seats, bags or upgrading an existing booking. Any additional payment must be made using a credit card. Credits cannot be used at the Airpoints™ Store, for Voluntary Emissions Contribution Programme contributions or to purchase other travel products such as hotels or rental cars.
Who: You can redeem part or all of your credit for yourself or someone else. If you authorise someone else to use your credit, they will need the booking details of your original booking to access your credit.