When is the best time to go trekking/hiking in New Zealand?

Tracks such as the Abel Tasman, Heaphy and Queen Charlotte Sounds Walkway located at the top of the South Island can be walked all year round.
However, those tracks at higher altitudes such as the world famous Milford Track, Kepler and Routeburn are closed in the winter due to snow. You must book to walk the Milford and Routeburn tracks, which are open between October and April.