I'm an ex-patriate New Zealander. Lived there from birth until I was 25, emigrated to the USA to get married.
One piece of information I'll share with you regarding Auckland, where I'm originally from, is that the climate is very very similar to that in the Bay Area. They even have similar foliage. Don't expect it to get too hot or too cold there. The further south you go, the colder it gets, seasonally.
The North Island is referred to as "the North Island", the South Island is referred to as "the South Island". There is a great deal of provincial pride in New Zealand, whether you're from the Waikato, Canterbury, Northland, or wherever. I noticed that Aucklanders are regarded as a little snooty by the rest of the country, but it's mainly because Auckland is the biggest city (and has more people living in it than the entire South Island) and therefore the most cosmopolitan. Then again, it also has the highest crime rate, but in New Zealand that's still relative.
I hope you enjoy your trip. Planning is a lot of fun, also. Don't be afraid to spend a lot of time in one or two places - a whirlwind tourist spin might not be as satisfying. For a country as small as New Zealand, there is an awful lot to see and do.
Cheers,
Cam
PS: Whatever you do, never confuse us in any way, shape or form with Australians. Sure, we may sound alike, but that's like confusing United States residents with Canadians. Bound to cause a little trouble.