Three Sydney teenage girls have been hit by a runaway school bus with no driver on board.

One of the three 15-year-old girls suffered only minor injuries, while the other two are in a stable condition in hospital after the bus stuck them as it rolled 40 metres down a hill on Quarry Road, Dural.

The bus rolled away from an assembly area outside the school after school ended at 3pm on Wednesday, CareFlight Helicopter director Ian Badham said.

The bus came to rest after crashing into a tree.

"Suddenly this very silent bus has just rolled down and fortunately pushed them away with a glancing blow," Mr Badham said.

Mr Badham said paramedics were concerned when they spotted a number of school bags under the bus, but all students were safe.

Police have launched an investigation.