The recommendation I got before going to NZ was to do the trip opposite to most travellers for the best deals. My partner and I started in Christchurch, got a van directly from another backpacker, and sold it in Auckland after putting it on gumtree within 24 hours.
Hope that helps!
This last post is incredibly true. They even have ridiculously priced car and van rentals moving them between cities (always north). Check Trade Me for private dealers and beware the sketchy car lots that look like all their cars were stolen!
Several of the big rental companies actually ask drivers to drive some of their camper-vans from Christchurch, over on the ferry to Auckland over a period of no longer than three days for free around Easter time. Not great if you want to explore the country but still a useful way to get from Chch to Akld.
To all of you guys who have had experience with those scammers:
I have being screwed by those guys and their names are Nina Tasev, mastermind of all. The next one is her husband, small time crook: Goran Tasev and Sam. I am building their portfolio for all their scams simply to protect other innocent people. All I can tell all of you, if you would like to contact me and give your impute, please don't hesitate to do so at any time.
In mean time for those who didn't see this, I wish you to enjoy: