Hi, I'm form NZ. It's usually pretty warm around Christmas. However, it is still not 'properly' summer so there is still a chance of rain, (although our weather is starting to clear up now!).
You will get away with light clothing but make sure you pack some warmer things just in case. Also, if you are going to be on a boat it could get quite chilly at night time.
ps the Auckland region (if you are going there) is famous for its fickle weather; it could look like a sunny day in the morning, but be raining by lunch time!