The other dynamic is that there is a steady flow of used cars from Japan to NZ. Laws in Japan make it attractive to sell a car with 30K miles rather than keep it, resulting in a glut of cheap used Japanese cars which are exported to the RHD countries that allow it (mostly NZ and Australia, but also to South Africa; UK doesn't let them in). At least this was the case a few years ago when I was last interested in this. Some of these japanese cars are in fantastic shape (like new), some have had a very hard life (mostly idling, and rarely over 20 mph). All are very nice cosmetically.
Rental car cost is very unpredictable in NZ, it goes up and down a lot. I've found that you cannot predict the pricing, even within the same company (a small change in the rental can be a big difference in price -- a rather absurd system). Sometimes the majors are cheaper than the "used car rental" places (which shouldn't be, but is). Sometimes it costs double.