Self-drive is a great way to explore New Zealand. Even those used to driving on the right won't be too phased by the keep-to-the-left conditions in New Zealand. Roads are generally of a good standard and traffic, except in the centre of cities, is light.
Car rental outlets are numerous and there is considerable difference in rates between operators. To some extent you get what you pay for but there are some operators with reasonable rates and good vehicles, service and backup. We did some investigation and chose to use Maui, a large New Zealand operator with depots in both the North and South island.
You need to decide if you want a regular car or a motorhome. Both have certain advantages. We opted for a sedan as we planned to stay in accommodation and needed to travel large distances in a short time. We were happy with our decision.
You need to be 21 to hire a rental car and show a current driving licence and/or an international licence. Australians just need their local licence. Seat belts must be worn by all passengers. Traffic rules are based on international standards but you need to remember to keep left at all times, including roundabouts. You cannot turn left on red lights. Speed limits are 100 kph on the open road and 50 kph in built up areas.
This is definately an option for most people. The cost is reasonable and the flexibility you get from your own wheels is great. We strongly recommend it.