A man is in a critical condition in Australia following a crash, which left his truck dangling precariously from the bridge on a busy stretch of the city's road network. Melbourne Fire Brigade Assistant Chief Officer Adam Dalrymple said it appeared the truck driver had swerved to avoid a car that had suddenly veered in front of him.

"From all reports the car veered across and the truck took some evasive action and that's how the incident began. It's a pretty intense, horrific sort of incident" he told reporters at the scene.

The driver of the truck, believed to be in his 30s, was ejected from the cab - falling onto the street running below Bolte Bridge. The man was found by a nearby factory worker, who comforted him until an ambulance arrived.

"I just made sure he was conscious and kept him calm until paramedics were able to take over. He kept saying he was going to die" he told Channel Seven news.

The driver had suffered serious head and pelvic injuries, an Ambulance Victoria spokeswoman said. He remains in a critical condition at the Royal Melbourne Hospital.

A witness told Channel Nine "There were no signs of obvious trauma. No blood or anything like that. No bones protruding I saw the truck as well, hanging upside down, and couldn't believe he was still conscious."

The other driver, a man believed to be in his 20s, suffered minor injuries and is in a stable condition in hospital.

Cranes were brought in to secure and then move the dangling truck. The crash also cracked a brick wall on a building beneath the bridge.

Fire crews are examining the building to make sure the damage does not cause it to collapse.