It's available by boat at City Wharf. The ticket is about 19 NZ. There are no hotels on the island, so they usually go back and forth on the same day. You can rent a bicycle as a means of transportation when you go down to the island, because buses usually wait for a long time, and the island is very large, so walking is not possible. Be sure to apply high-power sunscreen. The hot sun can burn you in a few minutes.