POLICE are appealing for witnesses as they investigate an assault that occurred in Wallan on Tuesday night.

Investigators believe the victim stopped at a service station on the Hume Freeway at 7.30pm and was at the bowser when he got into a verbal altercation with a truck driver who had also stopped at the service station.

Police believe the truck driver struck the victim on the side of the head with a metal pole before getting back into his truck, believed to be a white semi towing a single trailer, and driving north on the freeway.

The truck driver was described as Caucasian in appearance, bald with a beer belly and in his 40s. He was wearing shorts and a grey t-shirt at the time of the assault.

The victim a 24-year-old man from Fawkner, was taken to hospital with serious head injuries.

Police have established a crime scene at the service station and investigators are urging anyone who witnessed the incident or has information that could lead to the identification of the truck driver to contact Crime Stoppers by calling 1800 333 000 or submit a confidential crime report at www.crimestoppersvic.com.au.