DD, we have done NZ twice, SIL over there, but we only stayed with her for a few nights, hired a vehicle both times and like Sam stayed mostly motels and some camp sites, no plans as such just decide where you want to head the next day and check out motels & camp sites on maps, word of caution though you can check the accommodation before deciding, so if it wasn't up to scratch just moved on. On the plus side the motels where much more to our liking, one near Rotorua we had a complete house with underground parking, stayed 2 nights!!!
First time top half of North Island up to Cape Reianga and second time the bottom half down to Wellington.
Nephew moved to South Island, yet to venture to that one.