The incident occurred on Stawell-Marnoo Road, six kilometres from Marnoo, about 9.30pm on Sunday.

A fatal crash on the Western Highway meant the route was used as a traffic detour.

Stawell Ambulance Victoria team manager Wayne Rice said a B-double truck, loaded with potatoes, was headed towards Ballarat when a Smart car tried to overtake it.

"The 33-year-old male driver from Adelaide had overtaken the semi, but pulled in too early, clipping the front right-hand side of the semi, which sent him into a spin," he said.

"The Smart car hit the safety railing, and was then hit by the semi-trailer.

"It spun out of control across the bridge.

"The front left-hand wheel and steering assembly were torn from the vehicle."

Mr Rice said the man escaped uninjured.

"The driver opened the left-hand side car door and stepped out of the vehicle unscathed," he said.

"There was not one cut on him.

"I could not believe it.''

Mr Rice said the driver's side door was pinned shut because of the angle the car had come to rest on.

The truck had panel damage to the front left-hand side, which caused the front left-hand side tyre to blow out and split the rim in half.

The truck driver was also uninjured.