Police search for truck driver after man killed in suspected hit-run on Melbourne freeway

Police are searching for a truck driver after a man was found dead on a busy Melbourne freeway overnight in a suspected hit-run.

Police believe the Thornhill Park man had been involved in a crash shortly before 11.30pm, prompting him to pull over on the Western Freeway near Truganina and get out of his car.

The 23-year-old man was then struck by another vehicle, which failed to stop at the scene, police allege.

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"Investigators believe the man exited his car and was struck by another vehicle which failed to stop at the scene," police said.

Police have spoken to a witness who claimed the victim had been driving erratically in the lead-up to the incident.

They said the witness saw the man's car crash into wire rope barriers before the car spun out of control.

The man then exited the car and was standing in the middle of the major arterial road when he was hit by oncoming traffic.

A female motorist drove past and called police.

"She swerved to avoid that person, however it looks like a vehicle following her may have impacted that person," Detective Senior Sgt Christian Von Tunk said.

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He said the driver didn't stop to help.

"We've got to understand that they may not know they've struck someone.

"There could be some damage consistent with striking a person, but it might be very minor as well."

Police identified two trucks who were travelling through the area at the time of the collision that they said could assist with the investigation.

One of the truck drivers was traced to South Australia and is currently speaking with police, while the second hasn't been found.

Major Collision Investigation Unit detectives are investigating the incident.

Anyone who witnessed the crash or who has dashcam footage or information is urged to contact Crime Stoppers on 1800 333 000.