You could also consider Aeroplan. Gold status after just 35,000 miles, which lasts up to 2 years if you make the 35k part way through a year. There's no joining fee, miles don't expire, and you can join from anywhere in the world. The mileage cost for most *A rewards is slightly higher than United, however.
Another advantage is that Aeroplan awards miles for each sector flown (with a minimum of 500, in general), where as NZ air points only awards points for the entire one-way journey.