from Wellington, through the Cook Straight into Picton, 3 1/2 h on the big Ferry, collecting first impressions of the South Island and the Marlborough Sounds, a truly impressiv entrance, when the Cook Straight is calm.you arrive in Picton and can take a Bus to Nelson, or you rent a car.
timetables and fares at the webpage below.
and why not do it style, rent an oldie, its fun too. seriously: when you arrive in Picton, all major Car Rental Companies are at hand. you could book from Wellington and pick then up in Picton in no time.
just don't make the mistake to rent in Wellington and then have to pay the fare for the Ferry..160.- bucks out the window.
Budget, Avis, Hertz, take your pick
Nelson has a small airport with daily flights from Auckland, the gateway to New Zealand as well as from most other domestic airports, both small & larger. As always book early for best rates.