This is a short guide from a New Zealand kitesurfer.
Kites: Bring a 12-14m kite, 9-10 and a 5-6m if you can. You'll probably get to use them all over the course of a month of travelling.
The water around the South Island can be cold so make sure you have a good wetsuit, booties, a cap and even wetsuit gloves.
Cold as glacial bodies of water, 4/3 wetsuit country but they get good afternoon thermals (coming down the Alps towards the plains). Big winds in the famous NW'ers too. Pack a small kite.
Plenty of swell but not really that great for wind. The mountains are right on the coast and keep the winds relatively light even in a howling onshore, a SW seabreeze can set-up which provides some side to side off kiting around Westport at Carters Beach but to be honest I'd just enjoy the scenery or go surfing.
Fairly consistent N-NE seabreeze depending on the prevailing weather patterns. Only around 15 knots so you will be on big kites but the spots and consistent winds make up for it. Occasional nuking SW comes down the valley too.
Spots here are Tahunanui in town, crowded by NZ standards but has an emptying estuary "Magic carpet" so get's a good 5 knot boost on wind speed. It can get choppy. It's a cool beach though because you can get a coffee at the launch site, easy rigging and launch areas.
Marahau is about an hours drive away and the gateway to the Abel Tasman National Park. Huge sand flat and at mid tide has sandbars to jump, shallow clear water and channels in between. This is probably the most unforgettable place to kite surf in NZ, amazing scenery, golden sand and super fun kiting. Gets around 15-17 knots in the seabreeze. Usually 4 or 5 kiters out when it's on.
Golden bay is a giant shallow bay and sand flats, loads of spots and gets way more wind than the Nelson side of Takaka hill- happy hunting over there.
Enjoy but be prepared to kite alone!