A man has been taken to hospital with head and chest injuries after a large brick was thrown through the windscreen of a truck on the Monash Freeway early this morning.

The 23-kilogram bluestone brick was thrown from a footbridge near the Warrigal Road exit in Oakleigh and struck a passenger in the truck about 1:00am (AEST).

The truck was travelling around 100 kilometres per hour at the time.

Sergeant Andrew Douglas says it is "mindblowing" that no one was killed in the attack.

"It was just very fortunate that [the brick] actually struck just in the middle of the two passengers," he said.

"It has not hit them flush in the chest, because if that was the case we would not be talking about an injury; we would be talking about a fatality."

Police believe the brick was taken from a drainage gutter nearby.

The victim was taken to hospital with a fractured collarbone, fractured left arm and possible facial injuries.

Police say it is the second time a rock has been thrown at a vehicle from the overpass in the last week.

A similar sized rocked hit a taxi travelling along the freeway on Saturday morning.

No one was hurt in that incident.

Senior Sergeant Gary Maddern has appealed for any witnesses to the attacks to come forward.

"I just hope that the people who committed this act realise that it is quite serious," he said.

"If caught, they are facing serious criminal charges."