Rental car or campervan is the most practical option for most visitors. The road network is good, traffic outside the main cities is light, and many of the best places are not reachable by public transport at all. New Zealanders drive on the left. Roads through the mountains can be narrow, winding, and subject to rapid weather changes — don't underestimate distances or drive when tired. International licences are valid for up to 12 months.
Campervans are popular for good reason: New Zealand has excellent holiday parks and Department of Conservation (DOC) campsites in scenic locations across both islands. Rules on freedom camping (overnight parking in public spaces) are set by local councils and have tightened in recent years — check the Campermate app or camping.org.nz before assuming you can just pull over and sleep.
InterCity is the main intercity bus company and covers most of the country with daily services. It's slower than flying but a reasonable way to get between major destinations without a car. Backpacker-oriented hop-on/hop-off bus passes (Stray, Kiwi Experience) are popular with budget travellers and include activities along the way.
Trains are scenic rather than practical. There are only three long-distance routes: the Northern Explorer (Auckland–Wellington), the Coastal Pacific (Christchurch–Picton), and the TranzAlpine (Christchurch–Greymouth). Worth taking for the views, but they don't form a network you can use to get around the country.
Flying makes sense for the Auckland–Wellington–Christchurch triangle and for reaching Queenstown. Air New Zealand dominates domestic routes; Jetstar competes on the main corridors. Book ahead for the best prices.
Ferries cross Cook Strait between Wellington and Picton (the InterIslander and Bluebridge) several times daily. The crossing takes 3–4 hours and the scenery entering the Marlborough Sounds is genuinely good. Book ahead in summer and around public holidays.
Within the cities, Auckland has a reasonable bus and train network and a ferry service across the harbour. Wellington is compact and walkable with a good bus system. Christchurch rebuilt its public transport after the 2011 earthquake. In smaller towns, a car or bike beats waiting for an infrequent bus.