Ambulance Victoria has launched an investigation after a man was declared dead in a car crash, laid out for collection by a coroner's van - and then found to be alive.

The driver, a 30-year-old man from the Melbourne suburb of Hawthorn, was trapped upside down in the wreck for several hours after the crash on the Western Highway at Bacchus Marsh in the early hours of Sunday morning.

Tow truck operator Trevor Oliver says there were three ambulances and six paramedics on the scene, and they worked on the man for about 20 minutes before he was declared dead.

But Mr Oliver says he saw the man moving after he was pronounced dead - even after his 'body' was pulled from the wreck and laid out under a tarpaulin.

"There was some movement in the vehicle from the person who was trapped," he said.

"There was some leg twitching and that going on on the ground whilst he was under the tarp."

Mr Oliver says that after the ambulances left the scene and police began their investigation, the man remained in the vehicle for about an hour.

He was eventually removed by State Emergency Service (SES) workers who placed him on the ground for about 30 minutes while waiting for someone from the coroner's office.

"The coroner's contractor turned up to remove the body and it was when they went to remove the body into the coroner's van that they noticed that the driver had a very weak pulse," Mr Oliver told ABC local radio.

"They called an ambulance back and they turned up and transported the patient to hospital."

The man was taken to hospital in a critical condition.

Body movements

Steve McGhie from the Ambulance Employees Association says it is not unusual for there to be movements from people who have died.

"There can be still body movements and noises and different things that come from bodies, and so that is possible," he said.

"Paramedics would check the pulse, would check the patient's pupils, they would monitor the patient to see whether there's any heart rhythm."

Ambulance Victoria spokesman Simon Thomson says an investigation is now underway.

"This man was extensively trapped. He was upside down and the roof of the car was against the crash wires on the freeway," he said.

"In the 13 years I've worked for Ambulance Victoria I've never heard of a case like this.

"We attend over a half a million cases every year so obviously it's exceptionally rare."

Professor Russell Gruen, a trauma specialist at The Alfred Hospital, will oversee the Ambulance Victoria investigation.