Although infrequent, there is a great bus network serving part of the island (note that in much of the eastern part not all the roads have been sealed, hence no bus service). All buses converge on the foot passenger ferry at Matiatia and pass through Oneroa. From there, points served are basically Ostend/Blackpool: Rocky Bay/Palm Beach: Onetangi. NZ$1.80 will cover Zone 1 (Oneroa) up to NZ$3.70 for Zone 3. You can get a full day pass as well.
The Fullers buses run around the island very regularly, and of course they meet all Ferry sailings to pick people up.
You don't have to stand at a bus stop on the Island for long before one comes along. They weave their way around all the popular spots, and residential areas.