I travelled NZ a few years backmainly in the Winter and Spring. While I would hesitate to recommend clothing, heres my 2 cent comment on the weather.
I was mainly in the south island where even in winter I found the days relatively warm and dry, but the temperature falls to below freezing at night. Having said that if you go to the fjoirds onthe Southwest of the island, the rainfall is around 7 metres a year!
The trouble is NZ is such an amazing country in terms of scenary, climate and culture its hard to know where to begin. I would be prepared for every scenario.
I'll be waiting enviously to hear about your exploits - enjoy!
joblot