Expect to get very wet and very cold in the South Island in July and August. It's peak for winter and New Zealand will show you that! It can be clear and sunny yet still be close to freezing. On the other hand, at higher altitudes, heavy fogs ("pea-soupers") are common, again freezing. Expect dim light when the heavy rain comes and be ready for bursts of sunlight coming through clouds between "stream showers" (an euphemism for chucking it down every hour or so). Personally I've only ever been more concerned with keeping warm and dry and not worrying about photographic capacity — at least until the rain stops!!

What would you be shooting at 1/1000 BTW?? I'm more commonly down to 15 to 30 seconds...

Dunedin and its southerly neighbour, the English-like Christchurch, are both excellent places to chill out (or warm up) when the weather keeps you grounded. Maybe put contingency plans in place for stops in places like either, and also consider a side trip to uber-trendy and photographically-endearing Queenstown (but expect to pay a premium in QTN in winter because it's the peak ski season).