Connectabus managing director Ewen McCammon said the 36-seat bus travelling between Queenstown and Arrowtown, with about 25 people on board, failed to stop at the bottom of a hill near Edith Cavell Bridge, Arthurs Point.
The vehicle came to a halt on Moonlight Rd reserve after hitting a tree.
Mr McCammon said two passengers on the bus were taken to Lakes District Hospital for treatment for minor injuries. Another two received treatment at the scene for shock.
The driver was shaken but uninjured.
Mr McCammon said it was raining hard at the time and a slippery road surface would appear to have been a contributing factor.
''We are now working with the investigating authorities to establish exactly what happened and why,'' he said.
Constable Terry Wood told the Mountain Scene the driver said he was braking, but the bus was not slowing and he could hear it skidding.
The bus had then bounced off the left-hand kerb and the driver, ''seeing little other option'' aimed for a large pine on the banks of the Shotover River.
