What do you want to see? Like the countryside or the rain forests, or the garden city (christchurch)?
I live in Christchurch, I can say would be best to come in the spring/summer (unless your into ski'ing) that way you can see all blloms and pretty stuff. I reccomend a trip to hanmer springs and do a bungy jump! If your one of the "wow a Skyline" group, you will love it here! We can get GTRs for like $30,000NZD!
Edit to add stuff