Yeah, there's surf in the summer. I did a trip round NZ a couple of times when I was in university (actually got kicked out of uni for taking the student loan two years in a row and pissing off for 3 months

).
You'll definitely need some transport. Free camping is easy enough in most places. You can pick up an OK van for $3-$4k, insurance isn't a requirement, but worth having and it's cheap. Apart from Auckland, there's bugger all people about. 4m total population, 1m of them in Auckland. There are only 1m people living on the whole of the South Island. You can easily go to places where there is nobody for miles around.
Generally, the further north you go it's warmer, but can be wetter, and the East Coasts tend to be less windy with less swell than the west coasts, but both coasts have great waves.
It starts to warm up in November, summer goes through until April ish.
Places to check out - North Island East Coast - Coromandel, East Cape, Gisborne (great waves), Mahia. North Island West Coast - Raglan, Taranaki (great waves). South Island - East Coast - Kaikoura, Banks Peninsula, Dunedin. South Island West Coast - heaps of spots, usually massive, often onshore, wild and rugged. Worth a couple of weeks of exploring even if you can't surf.
If you go to
www.surf2surf.com they sell a decent guide book, which would be a big help in planning a trip.
You definitely need some transport though, and some bug repellant. NZ has these things called sandflies, that really really f*ck you off, you get eaten alive all the time.
If you want to know about anywhere specific, let me know. I've surfed my way round both islands a couple of times.
It's a beautiful country, with a lot more than just surfing to keep you occupied. The tramping is great too. And if you like fishing, you'll be a pig in sh*t.