Gah! I got all excited, was going to wax poetic about the wonders of the South Island, re-read you post, all hopes dashed mercilessly! I struggle on.
I haven't been to the North Island much, but Wellington is great. Bad weather is almost a certainty. Lots of people (at least in NZ terms? I can't judge by German standards). Lots of interesting things around. If you are into architecture Wellington has a lot of early-ish postmodern work. If you want any gifts for anyone, and happen to be walking down Cuba Street (one of the main retail areas/pedestrian mall), give this shop here a visit.
http://www.metshop.co.nz/ Lots of fun nonsense.
How are you travelling? How much freedom do you have? For NZ you will need to hire a car to get the most out of things.
Auckland is just sprawl, and Palmerston North is pretty boring too.
If you want to buy film while you are here, it isn't hard to find. All camera shops will have some selection (maybe missing out on some favourites, eg. FP4 is rarer, more expensive), some shops are better than others. As with everywhere.
Edit: Oh, and if you were to go to the South Island, give the Abel Tasman National Park area (top of South Island) a good look, wondeeerrrrffful landscapes around there. Also Queenstown (expensive!), and yes, Arthur's Pass is great. Around the bottom of the South Island, the Catlins are just stunning.
Christmas in NZ is great, summer Christmas is the only way to have it. Lots of barbecuing.