The 19 year old of Werribee had surgery for a broken arm in the Royal Melbourne Hospital. The truck driver, a 50-year-old Bacchus Marsh man, escaped uninjured.

Acting Sergeant Paul Gatty of Melton police said it appeared the Werribee-bound teenage driver veered into the front of the Melton-bound truck, which was carrying four vehicles, in Hopkins Road between Boundary and Griegs roads about 8.10am.

"The driver of the Daewoo has been ejected from the vehicle. It was fairly chaotic," Acting Sergeant Gatty said.

The roads were wet, there was a light drizzle and there was debris scattered over 400 metres, he said.

"The injuries are serious and he's undergoing surgery as we speak."

Ambulance Victoria paramedic Bruce King treated the victim at the scene.

"We put a neck brace on given the potential for spinal injuries. He suffered significant injuries to his arm, including several fractures, so we fitted a splint to his arm.

"It was a cool morning and the man's body temperature dropped significantly while he was lying on the road, so he was wrapped in blankets, and put into the ambulance with the heater on, to raise his body temperature."

Several drivers skidded when the vehicles collided, and traffic was thrown into chaos with the stretch of road closed for four hours while emergency crews and police cleared the scene.

A Melton traffic management unit spokesman said investigations were continuing, but he believed the wet road did not play a part in the crash.