The fireman was trapped in the fire truck for more than an hour before being cut out and taken to the Launceston General Hospital.

Three others in the fire truck were injured but neither drivers were hurt.

The accident occurred at 8.15 pm when the west-bound fire truck, carrying five people, attempted a U-turn and collided with an on-coming semi-trailer, also travelling west, as it came over the crest of a hill.

Police say a white 2007 Freightliner Argosy prime mover and loaded tri-axle trailer was being driven in a westerly direction by a 33-year-old South Launceston man.

"A red 2006 Hino dual-cab fire truck was also being driven in a westerly direction by a 50-year-old Summerhill man with a crew of four on board," a police spokesman said.

"The fire crew were responding to a reported motor vehicle accident on the Bass Highway. The accident was on the opposite side of the highway which resulted in fire truck having to drive past the scene in order to turn around.

"The fire truck, with emergency lights activated, commenced a right U-turn.

"The semi-trailer travelling in the same direction has observed the fire truck commence the u-turn and has applied heavy braking and taken evasive action before impacting with the fire truck.

"On impact the fire truck has been pushed sideways about 25 metres before rolling onto its roof on top of the Armco railing on the northern side of the roadway.

"Both vehicles have continued a further 10 metres before coming to rest. The prime mover was facing a westerly direction on the eastbound road verge and the fire truck was upside down facing east resting on top of the Armco railing."

Police say both vehicles sustained severe impact. The three fire crew who sustained relatively minor injuries were also taken to the Launceston General Hospital for treatment.

The east-bound lanes of the Bass Highway were closed for an hour with traffic diverted on to Illawarra Road. The crash is still under investigation.