Although not the biggest country on Earth, New Zealand still has an excellent network of flights between almost all the main destinations, and these are definitely the best way to get around, not least because of the spectacular views you get over the varied landscape as you fly above it.
If you plan to see a lot of the country, there are two main ways of getting around under your own steam: either hiring a car, and staying at hotels and guest houses as you travel, or alternatively, and probably more liberating, hire a camper-van and just stop wherever you feel like it.Getting between the islands can be done on the many ferries that pass through the Cook Straits. Some car hire companies will require that you leave your car on North Island and pick up a new one on South Island and vice versa.
New Zealand's roads are generally very good, and make travelling easy, driving is done on the left and there are plenty of campsites to stop at.