Depends what you are in to Andi. If you love the nature, walking, beaches, etc then there is no doubt you should go to the South Island. Fly directly there and hire a camper if possible.
If you prefer the bigger cities and shorter drives then go to the North Island and go to Taupo, Rotorua, Bay of Plenty, Coromandel, Auckland. Still pretty, but not as dramatic as the south. You can hire campers for one way drives.
January is a good time of year.
Can I come too?