It's a great time of year for camping, except for the fact that it's the Christmas / New Year holidays so campgrounds will be at their busiest. Still, this is NZ we're talking about so it's all relative. The most crowded places will be the traditional new year spots like Bay of Islands, Whangamata, Mt Maunganui, Kaiteriteri, Wanaka etc, and since I doubt you're coming to NZ to be surrounded by drunken adolescents you might want to choose your sites with that in mind. NZ has many fantastic places to camp - go for it!