The vehicle involved is understood to have left the road and rolled north of Casterton about 8.45am.

A man who claimed to be a passenger in the vehicle went to Casterton hospital for treatment.

Police are making inquiries into the identity of the driver and the cause of the accident.

Meanwhile, Portland police have warned residents to lock their vehicles after another theft from an unlocked car.

Detective Sergeant Steve Brown, of the crime investigation unit, said there had been a string of thefts from unlocked vehicles in Portland during the past few weeks.

A wallet was stolen from a car in Stornaway Court in south Portland earlier this week.

"We're urging residents to lock their vehicles and secure their valuables. People are just making it easy for opportunistic thieves," he said.