Climate
New Zealand is comprised of two main Islands, North and South. Of the two, the South Island is usually colder.
Winter
During Winter the South Island can easily drop to a little bit below zero degrees celsius. So during winter you need to be much more careful when riding on the road in the early morning or late in the day (when the sun hasn't had much effect) because of the dangers of black ice. As a result if your coming to NZ and touring the South Island during Winter plan your ride accordingly.
The North Island can also get down to zero degrees celsius, but doesn't drop too much further down. An average day time temperature might be 10 degrees, or perhaps a little warmer.
Regardless of which Island your in you need to consider how to deal with helmet fogging (such as using a pinlock visor, a breath guard, a chemical treatment, etc). Wearing a lined jacket and trousers is highly recommended, was well as something to keep the wind off your neck.
Summer
During summer the day time temperature is often 20 to 25 degrees Celsius. You'll want to remove any linings you have from your jacket or trousers or you may end up getting too hot. Riders often choose to wear kevlar lined jeans as they are much cooler than leathers or cordura. The safety choice is yours.