I posted a message on this a year or so ago, but here are the details: We did used Stand by cars, who operate this service in both NZ and Australia.
http://www.standbyrelocs.com/default.aspx
They always have a list of cars and campervans available for relocation, most are from Christchurch to be returned to Auckland, but for instance right now there is an Auckland to Christchurch deal listed which is $1 a day for a car, with free ferry included.
We have done it both ways, took a 4 berth with shower down to Christchurch and then brought a 2 berth back up a couple of weeks later. We loved it, had plenty of time to stop over , the 2 berth was best for us, as it enabled us to do more "freedom camping" parking on beaches and road side paddocks etc. it was a cheap and brilliant holiday. With the 4 berth we tended to stay more in organised campgrounds like Top 10.
Make sure you have a very good look at the condition of the vehicle because they turn these vehicles around very quickly with little more than a glance at it, we found a hairline crack in the windshield just starting from the bottom that we and they had missed, but we phoned them as soon as we noticed it and they were very good about it.