Gorgeous sunny days all round, for any activity that you may choose (even the waters are relatively warm for snorkeling and diving). The cities are festive, and the great NZ outdoors are a sheer experiential delight. Expect lots of tourists, expensive flights, hotels full and travel operators maxed out, unless you book at least 6-8 months in advance. June to August is peak skiing season; book at least 6 montsh in advance for resorts.
Not as crowded as the peak season, but still plenty of reasons to visit New Zealand. The autumn and spring periods are a time of lesser crowds, and the weather perfect for those who prefer cooler temperatures. September and October are months of snow melt, making for some spectacular white-water rafting.
Both islands are in the grip of winter, but yet there is something to be said for the sheer beauty of New Zealand winters with virtually no crowds and unspoiled landscapes. However, do note that many excursions and activities may shut shop, so please check with the operator.