It really depends on what type cars you'd like to rent. Big companies like Hertz offer new model cars with better service. Small companies offer 2nd hand Japan imported car. I have used the following company in Auckland for a couple of times. They also have office in Christchurch
In addition, if you have a Gold credit card in Australia and you use this card to book your flight (or pay at least half of your travel cost), you are automatically entitled to 90 days travel insurance in NZ. The travel insurance generally covers the high excess cost for rental car insurance (up to $3K). This is one way to avoid expensive excess insurance (all the rental cars have insurance but with high excess cost).
I generally don't recommend camp van type of vehicle. It seems like you can save accomodation cost. However, most of the time, you need to park the van inside a holiday park over night (if you want to use shower or other facility or for safety reason), they will charge you a fee. In addition, camp van uses more petrol and some times the cost is also associated to the milage you have travelled. Accomodation in NZ is not expensive. If you have four people traveling together, you can get a decent motel room for around NZ$30 per person. If you choose to stay in a backpack style holiday park, it is even cheaper and you also have the chance to meet girls from all over Europe. Swedish girls are very sweet...