New Zealand winters are the northern hemisphere's summer and are quite chilly. Remember to bring a large jacket and a few sweaters; gloves are also VERY helpful. The southern island is the only one that gets snow, so if you are planning to travel down south, be prepared.
Rugby is a very good way to get the blood flowing, but be prepaired to get muddy. I suggest wearing plain old shorts and a t-shirt, and to find people you are comfortable playing with.