No matter whether in summer or winter, New Zealand always is worth a journey. If it is a simple Economy car, a Motorhome, Camper, Sleepervan or a bike, the choice of vehicles is versatile in New Zealand. By the way, the free, individual camping with vehicles with shower/toilet and waste water tank still is allowed across most of New Zealand (subject to change). But many of the camp grounds are located so beautifully, that it is worth to using these.
Ferry crossings between the two islands should be booked a couple of days in advance, especially during the high season. Due to the numerous ferries, usually a passage can be guaranteed within a few hours only.

Our hint for skiers : Summer skiing in the winter time (from european perspective).
world-wide-wheels has been involved in camper rental and motorhome hire in New Zealand for many years.