When we first visited NZ we hired a campervan for the whole trip - over 3 months. The advantages for us were that we knew the van would be roadworthy, any breakdowns would be fixed by the rental company and that we didn't have the pressure of selling it before we left the country. You can get some very good deals for a long-term rental. We rented from
http://www.wendekreisen.co.nz/default.aspx. At the time they only hired older refurbished vehicles, which suited us fine.
If you do hire a vehicle, shop around for the insurance if you're going to take it. We found that some travel insurance policies covered motor insurance excesses, or there are specialist companies that sell policies just to cover car hire excesses. We've used
http://www.insurance4carhire.com/ when we've hired cars in Europe, but there are lots of other choices.