I wouldn't mind being in NZ from mid-March either! The autumn splendor of Lake Wanaka and Queenstown (South Island) should stir you into getting on the earliest plane out, with loads of film to burn.
The West Coast has been said to have its own weather system, and one that gives the most annoyance to travellers from around the globe. The East Coast is milder, but can still be utterly awful in bad weather. You have to be prepared for everything. I would rate the South Island a 13 out of 10 for scenic quality compared to the touristy North Island. The South has The Southern Alps (sunsets with these are phenomenal), the playground of Wanaka and Queenstown, the Coromandels, The Fjordland and its other-worldly spires of "crying mountains" (so much rain the peaks appear to be crying), The Catlins, an area of lush rainforest (where rain is the operative word in every respect) dotted with exquisitely pretty waterfalls must be seen. Extend your stay by another 8 weeks!
I don't really think you will be served properly relying on one or two films for this trip. When I went, I took with me 140 films, the majority Velvia 50, Velvia 100F, Provia 100F, ACROS 100 and NPS 160 (discontinued) print film.
There was a film processing store in Papanui, outside Christchurch that I used often in 2006; I believe it is now gone, along with many beautiful landmarks in Christchurch from the earthquake. I don't know (or like) Auckland very much. Maybe Sean here (the web techie) can tell you what facilities are around places like "Windy Wellington".