Crossing between the islands
The Interislander and Bluebridge ferries both run between Wellington and Picton across the Cook Strait, a crossing of roughly three to three and a half hours that passes through the scenic Marlborough Sounds on the South Island side. Both operators carry vehicles as well as foot passengers, and booking ahead in peak summer months is worth doing since sailings can sell out. Domestic flights are the faster alternative — routes like Auckland–Christchurch or Wellington–Queenstown run frequently and can be the better choice if time is tight, though they skip the ferry's own scenery.
Self-driving
A rental car or campervan is the standard way to see either island in depth — public transport between smaller towns is limited, and much of what makes both islands worth visiting sits between the main centres rather than in them. New Zealand drives on the left. South Island roads through mountain terrain, particularly the Milford Road into Fiordland, demand more caution and driving time than the map distance suggests; some passes can close temporarily in winter weather.
Campervans specifically
Campervan travel is genuinely popular in New Zealand and well set up for it, with a wide network of holiday parks and freedom-camping sites on both islands. It suits a slower, flexible itinerary better than a fixed-route trip, since accommodation and transport are combined into one booking.
InterCity buses
For visitors not driving, InterCity's coach network connects most major towns on both islands and is the main non-flying, non-self-drive option — slower than a car but a workable budget alternative, particularly for a North Island trip where distances between stops are shorter.
Getting around within cities
Auckland, Wellington and Christchurch all have local bus networks, and Wellington additionally has a small heritage cable car connecting the central city to the Botanic Garden on the hill above it. None of the three cities is set up as a car-free destination the way some European capitals are — a car remains the most practical way to reach attractions outside the immediate city centre on any New Zealand trip.