The weather in NZ can be described as a maritime climate which is influenced by the surrounding seas thus can change very quickly. The climate varies widely between the North and the South Island.
The northern half of the North Island has got a fairly mild climate and even in the middle of winter you will be cosy in a 4/3. In summer there can be days up North when boardshorts are enough and other days when you want to wear a 3/2. The further you go south, the colder it gets. Wellington to Christchurch normally requires some sort of steamer, in summer a 3/2, in winter a 4/3. For those who feel the cold more, maybe even with booties.
Further south than Christchurch, it’s getting seriously cold in winter. Summers are ok, but you want a 4/3 most of the time, in winter a 5/3 or 5/4 with hood and booties.
The inland lakes in the South Island can be a bracing surprise as many of the lakes are glacier fed. The air gets reasonable warm, but the water temperature reaches barely over 10 degrees Celsius (50 Fahrenheit) in the height of summer . Be prepared to wear a warm wetsuit, in winter you may prefer skiing or snowboarding!
There are plenty of websites to provide you with a good forecast for wind, waves, tides and the general weather.
Swellmap Swell maps for NZ
Windsurf.co.nz webcams on several beaches around Auckland
Surf2surf webcams nationwide
Metservice New Zealand’s official weather website
Metservice – Tide information
Metvuw Victoria University Weather forecast
NIWA – Weather forecast
NIWA – UVI forecast
NIWA – New Zealand climate overview