New Zealand has a mild and pleasant climate with an average temperate of 10°C in the South and 16°C in the North. Sitting straight in the path of the Roaring Forties, New Zealand is freshened by the cool winds from the Tasman Sea.
Weather conditions can vary quite sharply across regions, from extremely wet on the South Island to subtropical in the very North. The Southern Alps and volcanos on the North Island act as a barrier from these moist winds keeping the east coast dry and creating a wet climate on the west. This is more pronounced on the South Island, as the northern volcanos aren't as large as the Alps.
If you are looking for outdoor adventures then the warmer months (November to April) are for you. Summer is the time of the food and wine festivals and all the large sporting events.
However, if it's the open slopes and powder runs you're after then head there between June and August. The snow season is generally from June to early October on the South Island and if you're lucky it may snow on the North Island.