A truck passenger was checking the load on his vehicle was secure when he was struck and killed by a crane mounted to a passing vehicle on a freeway in Melbourne's west, police say.

Key points: The truck pulled into the emergency lane of the Western Ring Road and a passenger was checking the load

The truck pulled into the emergency lane of the Western Ring Road and a passenger was checking the load Police believe a 60-tonne crane truck was straddling the emergency lane when it slammed into the back of the smaller truck

Police believe a 60-tonne crane truck was straddling the emergency lane when it slammed into the back of the smaller truck The passenger at the back of the truck checking the load was hit and killed instantly

Police said the driver of the truck had pulled into the emergency lane on the Western Ring Road near Ballarat Road on Tuesday when he and his passenger got out to check the load on the truck.

While they were outside the vehicle, the 34-year-old male passenger from Hoppers Crossing was hit and killed by the passing crane truck about 7:20am.

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Paramedics said a man in his 20s and another in his 30s were taken to Sunshine Hospital with no obvious injuries.

A 31-year-old man from the Ballarat suburb of Wendouree was interviewed by police and released pending further enquiries.

Evidence from the scene suggested the smaller truck was parked "appropriately" in the emergency lane when the crash occurred, Senior Sergeant Gerard Clanchy told ABC Radio Melbourne.

"It's been struck to the rear by a 60-tonne crane truck that was partially straddling the emergency lane and the far left lane," he said.

"One of the occupants of the smaller trade truck has been at the rear of the truck adjusting his load and he unfortunately was struck and killed instantly as a result of the collision.

"Part of our investigation is to establish why the crane truck has been partially straddling the emergency lane.

"So we'll look at distraction, fatigue, the normal parts of our investigation."

Sergeant Clanchy said the crane truck was not wider than the lanes on that section of the Western Ring Road.

All lanes on the M80 Ring Rd have now been reopened.