Although the Southern Alps and Fiordland have four distinct seasons, it's not unusual for the mountains to get snow even in summer. If you're traveling in winter, check the weather forecasts and road conditions regularly. Mountain passes can close for short periods because of snow and ice and, even when they are open, can be very dangerous. The road into Milford Sound can close for days at a time because of snow or avalanche risk.
For skiing, snowboarding, and other winter sports, from July through September you can be assured of snow around Queenstown and Wanaka, where the ski scene is ushered in by the Queenstown Winter Festival. In the height of summer—from January to March—both resort towns are crowded with Kiwi holiday makers. To avoid the crowds, hold off until March or April, when the leaves start turning. If you're planning on hiking one of the major trails, such as the Milford Track, summer or early autumn are the best times, but you'll need to book ahead. This is also the best time to visit Fiordland National Park. Fiordland is soaked with rain year-round (it's the wettest place in the country), so there's little use trying to avoid the rainy season.