The weather over here in Dec,Jan, Feb is warm/hot as it is SUMMER!!! Bring your sunscreen (30 + SPF) and a hat!! East coast has the best weather (especially the North Island). If you are in Auckland it tends to change a lot especially in winter (May, June, July, August, September) i.e. rain one minute and then fine in 15 mins. As for transport, we do have buses to the out of the way places, but that tends to be infrequent even though it's pretty cheap. You are better off with a car Becky, but you'll probably be able to find some other people who would want to go to the same places that you'd want to go.
Good luck and have fun to all you guys that are coming here. I'll be in Auckland for New Years, but then I'm off to Aussie for 6 months for work.