I recently rented a car in Christchurch, New Zealand. I checked Pegasus, Hertz, Avis, and Thrifty. Hertz was the best deal and was only NZD$50 per day for a little Mazda 3 hatchback. The great thing about New Zealand car rentals is this -- the main insurance is included in the price. So you can decline all the insurance they may offer you -- the most you will pay in case of an accident is what they'd call the "deductible" here of $2500 (NZD), which they call something else there.
Be careful of the smaller car agencies. Pegasus charges by the km after you reach 150km per day. Hertz and Avis gave unlimited miles.
Lastly, if you stay in Christchurch the first night back and rent a car, Hertz and Avis have offices downtown. I saved the airport pickup fee for rental cars of $30 by picking the car up downtown and returning it to the airport.
One tip is that car rental agencies take your reservation on the web, but they will not charge you if you never show up. I don't abuse this, but I do sometimes make two reservations for different times with different car rental agencies if I am not sure of my times. I do try to cancel in advance but it is not a big deal if I do not. In the US at least -- and in New Zealand - I decline all the extra insurance that the car agencies ask for. In the US, my existing car insurance covers liability and all other stuff, and my credit card covers the deductible.