Driver fatigue and black ice are just two of the theories being investigated as the cause of a major accident at Mt Lambie early on Tuesday that left a truck driver seriously injured.

The Scania semi trailer was heading towards Lithgow shortly before 4 am when for an unknown reason it veered across the three traffic lanes of a relatively straight section of the Great Western Highway about 200 metres east of the Rydal turn off.

It hurtled across a deep roadside ditch on the wrong side of the highway before ploughing among pine trees in front of the ‘Canobie’ property.

Police, ambulance, rescue squad and fire brigade units were rushed to the scene.

They found the driver trapped on the dashboard area with his foot through the windscreen and crushed between the offside of the prime mover and a tree.

It took around two hours for the rescuers to free the 56 year old Seven Hills man.

An ambulance helicopter Rescue 21 landed in the darkness on the highway to assist in the rescue and later flew the badly injured driver to Nepean Hospital.

The rescue was carried out in sub zero conditions.

The accident resulted in the highway being closed in both directions for some considerable time.

A hospital spokesman yesterday described the injured man’s condition as critical but stable.