That's my favourite destination too!
Winter in the South Island presents a kaleidoscope of ever changing weather and very often quite hazardous road conditions, each development presenting unique opportunities (but especially how to keep dry), but you must be prepared. Dunedin, with its Scottish influence (particularly the beautifully manicured gardens at the railway station) will keep lovers of architecture occupied for hours, while the wife goes wandering off latté sipping at the Octagon. Everywhere is a cold place. It was around minus 5 degrees in August in Queenstown last year, the paths iced over and cafés were overflowing. Christchurch is the opposite to Dunedin: quite English and undeniably attractive in Spring. There is a good photo store at Papanui that still stocks film (as of my last visit in August 2009). It was hard to find any film elsewhere, including Dunedin.
Invercargill, dangling off the end of the South Island, honestly didn't leave a particularly favourable impression with me on a day so dull, grey and dark that everybody was driving with their lights on high beam at 1pm. The Catlins, en route, however, are absolutely worth a day or two walking to explore several notable waterfalls (McLean, Matai, among), but you'll doubtless get very cold and wet.
You didn't say where you're flying into? Dunedin? Christchurch?? If flying into Dunedin, come down the (hilly) hinterland along the coast, stop in Christchurch down a bit to ogle at the Moeraki Boulders (best at sunset or sunrise), brekky at Fleurs Restaurant (a favourite haunt of Rick Stein), inland to Queenstown for a couple of nights; pull into the campa park in Robins Road for fast foot access into the town centre). The wharf area and the gardens are always photogenic, and night photography can be quite exciting — nightlife is big time in Q-Town. It's my pick for an extended stay, just that it can be expensive if not in a hired campa of backpacking.
Trans-Alpine has quite jaw-dropping (and gravity defying) scenery; take some reasonably fast film for shots 'on the fly'.
The Southern Alps, when they get sunlight, are a must-see. Head out to a little past Fox Glacier to Lake Mathieson and if in a campa, stay there overnight, then walk the few kilometres to the water's edge for truly, madly, deeply, bloody marvellous reflections of the Southern Alps in perfect symmetry in the water. Get there early: hundreds of others will have heard about it before you! :rolleyes:
Be prepared for some sunny days but also cold, wet and dull days. Remember it's not sun-drenched Tahiti where Velvia 50 can thrive with a Polariser on the front. Probably best to have a fast lens at the ready but if it's really piddling down, grey and dark, put the whole caboodle away and wait for a break.
If you're prepared for whatever comes along, including the hordes of camera-toting tin-can tourists found wandering rainforest tracks wearing crocodile-skin slippers or pin-striped suits, the South will deliver. Enjoy.