Sitting just below the Tropic of Capricorn, New Zealand has a fairly warm climate and does not suffer from the climate extremes of continental countries. There is however a fairly high rainfall - especially in the southern tip of the South Island. The diversity of landscape means that climates can vary hugely across the country and many of the mountain ranges are snow capped throughout the year.
New Zealand's seasons are the reverse of those in the Northern Hemisphere meaning that its tourist season is too. Most tourists travel between December and February when the infrastructure is completely functional. Whilst you'll beat the crowds if you travel in winter, you'll also find that many of the national parks and mountain ranges are inaccessible. For a combination of good weather and relative under crowding, try a holiday in the autumn or spring - between March and April and October and November respectively.