From NZ's website
Airpoints.Dollars Expiry
Airpoints.Dollars are valid for 4-5 years. Each year, on your anniversary of joining Airpoints, all Airpoints.Dollars earned more than four years earlier will expire. Every time you use Airpoints.Dollars, the oldest ones are used first, keeping the Airpoints.Dollars due to expire to a minimum. To see when your Airpoints are due to expire, login or register for myairnz.
Your Airpoints.Dollars™ will not expire while you retain Gold Elite or Gold status. Should you move below Gold status, you will have until your next anniversary of joining to use any Airpoints.Dollars that would have otherwise expired. If your next anniversary falls within two months of you moving below Gold status you will have two months to utilise these Airpoints.Dollars.