Rarotonga is the only island in the Cooks with a scheduled bus service. Some other islands including Atutaki have buses, however these either operate as airport transfers for various hotels and resorts or as school buses.
There are only two bus routes in Rarotonga, two circle island routes - one clockwise and the other anti-clockwise. The bus goes everywhere you would ever want to go.
The main terminus for buses is Cooks Corner in Avarua, although all hotels have a bus stop. Buses operate hourly in each direction between 7am and 4pm. After 4pm, the service is much less frequent and after 10pm you'd better start walking.