Picton is the port for the passenger/vehicle ferry to and from the North Island. There are two companies - InterIslander and Bluebridge. Both leave from the terminal in picton.
Mum and I used the InterIslander. We decided to pay an extra $45NZ each for Premium Class. It was definitely worth it for the 4 and 1/2 hour trip. The ticket let us use the lounge with comfortable chairs, tea & coffe and snacks, a buffet lunch, wine & beer, wireless internet connection and our own staff.
The ferry sails often and can get you there and back to Wellington in a day. You can take your vehicle or just jump on yourself - public transport in Wellington is easy. The cost of passage is as low as $45 one way with the main sailings around 10.30am and 1pm.
I travelled mostly in Busses or with fellow travellers who had bought a Car here in NZ. when you arrive by Ferry, you can obtain information right there or at the Visitors Center in Picton, close by the Harbour