Ballan police Leading Senior Constable Ian Watson said the driver, an 18-year-old Melbourne man, sustained head and arm injuries in the single vehicle crash and lapsed in and out of consciousness as paramedics treated him at the scene.

He was taken to Ballarat Health Services Base Hospital in a serious but stable condition.

His passengers, a man believed to be the driver's father aged in his 50s and a 24-year-old woman, both sustained neck injuries and were also taken to Ballarat.

The father was also thought to have back and chest injuries.

Sen Const Watson said the cause of the crash was still under investigation but preliminary inquiries had ruled out speed as a contributing factor.

He said alcohol and drugs were yet to be dismissed.

Witnesses reported the Ford sedan was travelling south when it rolled three times on the roadside before coming to rest against a paddock fence about 3pm, Sen Const Watson said.

"Witnesses have said he did not go off onto the shoulder, so we are still trying to establish what caused the crash," he said.

"An off-duty paramedic from Melbourne was one of the first on the scene and was able to treat the driver before the ambulance arrived.

"They were all very lucky." said Sen Const Watson.

The accident occurred about 8km north of Ballan, near the intersection with Moorabool West Rd.

Traffic was briefly delayed as the three were treated by paramedics and Country Fire Authority volunteers from Ballan worked to clear the scene.