Police are searching for a white truck involved in a fatal collision in Melbourne's west that caused major traffic disruptions on Thursday night.

A white truck was travelling in the Altona-bound lane of the M80 Ring Road about 4pm when a ramp fell of the back of the vehicle. 

Investigators believe the ramp travelled across the freeway and collided with a car travelling in the opposite direction, killing the driver.

The truck did not stop at the scene, and was last seen travelling Altona-bound.

The deceased driver died at the scene and is yet to be identified.

As a result of the collision, major traffic delays are expected in the area for several hours. 

According to VicTraffic, all Greensborough-bound lanes of the M80 Ring Road are closed between Ballarat Road and Sunshine Avenue. 

All traffic is being diverted onto Ballarat Road using McIntyre Road to return to the M80 Ring Road.

Chris Miller from the Department of Transport and Planning said the delays could last all night as investigators examined the crash site.

"We don't know how long the closures likely to remain in place — but experience would suggest it will be several more hours to come," he said.

"Wouldn't be shocked if it was still closed at midnight."

Police are appealing anyone who witnessed the collision or has footage to contact Crime Stoppers on 1800 333 000 or visit crimestoppersvic.com.au