I normally rent through Hertz. They have several publically available CDP (corporate Discount rates). They aren't crash hot, but do save some $ sometimes.
Eg.
Qantas Frequent Flyer: 297302
Entertainment Book: 1441781
Your AAA (NRMA/RACV/RACQ etc) Eg. RACV 074269 or NRMA 074155
If you know some good CDP's then you can use them, but be aware that if you are involved in an accident then your use of the CDP could be questioned. Same deal when picking up the car.
Other companies like Avis et.al also have similar CDP's - you just have to hunt around for them.
There are also dedicated websites where you can book cars through a third party booking site (kinda like the last minute hotel booking sites, but for cars) although I can't recommend any.
Also, have a look at the following Jetstar page. There are some good (public) discounts for NZ rentals there:
Jetstar - Cheap car rental deals through Avis and Budget