Being in the Southern Hemisphere the seasons are reversed. Summer is Dec, Jan Feb. If you like heat and lots of people, then this is the time to come. Although what seems like a lot of people to New Zealanders seems quite manageable to most visitors! Christmas and New Year are difficult times to find accommodation unless you book well in advance. The shoulder months (November, March) are probably our favourites - the weather is not so hot, there aren't so many people, and the country is just as beautiful. April has also been a superb month in the last few years.
Most of what is on offer in New Zealand can be enjoyed outside summer too. If we forget about the beach and swimming, then the whole year opens up - hiking and cycling are often better when cooler, and good food, wine and golf courses are still enjoyable when you have to wear long sleeves.
For winter sports fans, our skiing is excellent and our heli-skiing is wonderful, and you can combine a skiing holiday with some enjoyable general tourism.