Even if you arrive in New Zealand in the summer, do not expect high temperatures. In the north, temperatures can reach up to 28 °C, unless something exceptional happens.
Personally I defined the weather in New Zealand as a mix of Italian and Irish weather. It is an island in the middle of the ocean, with a lot of rain and wind.
Fortunately, this island is in a more temperate area than Ireland, so the temperature is higher. Be careful with the sun rays , you are in an area of the world where the ozone layer is most compromised, the ultraviolet rays are really strong. In this area of the planet there is one of the highest incidences of skin cancer, so take precautions.