One man is on life support after an open-top military truck overturned during a defence exercise at Holsworthy base in south-west Sydney yesterday, leaving injured soldiers strewn on the ground, police said.

Metropolitan crash investigation unit commander Inspector Steve Blair said officers were confronted with a ''horrific'' scene. All 18 people inside the vehicle were injured.

Two men were airlifted to hospital and 11 wounded were taken to various Sydney hospitals by road. The other five were treated at the base's hospital.

One soldier was still critical last night and next of kin have been notified. Others were being treated for cuts, bruises and broken bones.

The truck carrying the soldiers veered off the side of the road and rolled several times. Inspector Blair said it appeared speed may have been a factor. Defence has declined to comment on the possible cause of the crash, nor would it release details of the injured soldiers.

Inspector Blair said that police would be on the site investigating the crash for some time, and would be taking detailed photographs of the scene, looking for skid marks and examining the truck's brakes.

He said the crash was particularly horrifying because of the number of casualties.

''We are always saying that any collision is bad,'' the inspector said. ''We are lucky at this stage that no one has passed away.''

AAP