"I was in a bit of shock, I couldn't believe it, then it kicked in and I realised what had happened.

"It hit the lower part of the windscreen, so I could still see past it."

Mr Martin was forced to swerve, before pulling into the emergency lane. He ran across three lanes of traffic in a bid to catch the teenagers, but to no avail.

"I pulled up and adrenaline kicked it and I thought bugger it, I want to catch them so I ran across three lanes of the freeway."

The three offenders, who fled into the nearby suburbs, have not been caught and investigators are appealing for anyone with CCTV or dash-cam footage to contact police.

Mr Martin, who is from Modella in Gippsland, said he had lost work as a result of the incident.

"It's the inconvenience of losing half a day's worth of work sitting around, and to pay for the windscreen. But everyone has been great," he said.

"It could have been anyone, innocent motorists. Truck drivers are a big target. It could have got the driver's side or passenger's side window."

Detective Sergeant Ashley Ryan said police were searching for three offenders aged between 12 and 16 of Caucasian appearance who had thrown rocks from the footbridge across the Western Highway in Melton between Corburns and Station roads.

"One rock, which was substantial in size, between 10 kilograms and 12 kilograms ... struck the windscreen of a passing truck heading in a westerly direction. The youths then ran from the bridge in a northerly direction into a residential area," he said.

"Luckily the truck was able to pull over without incident and the driver was not injured.

"He's a bit shaken up, our investigators have spoken to him at length in relation to what's happened. Physically he's unhurt ... it's concerning because it not only could potentially cause serious injury or death, it could also cause an accident involving one or multiple vehicles."

Anyone with information is urged to contact Crime Stoppers on 1800 333 000.