Bring both 100 and 400 speed films, colour and black and white. If you run out, you'll just have to "suck it up" and buy some at Kiwi prices In Wellington, WPS is the best place for film.
As for places in the South Island, if you've got time, try and pop across to Stewart Island. Another spot that is often missed is the north western most tip of the South Island from Takaka round to Farewell Spit (many tourists get to Abel Tasman National park and avoid going over the hill.)
In the North Island, the southern coastal regions are quite spectacular, round Wellington and the southern Wairarapa coasts. You could look to head up on the west coast, say towards Wanganui, then head up through the Parapara's and along the Wanganui river. Then head on over to the central North Island plateau. From there, you could head over to the east coast and go round the via Gisborne and East Cape, Bay of Plenty and up to Coromandel.
Feel free to send a PM if you're stopping near Wellington.
(Oh, btw Poisson, if you're hoping to find the Coromandels in the South Island, you're going to be looking a long time.....)