Investigators believe speed may have been a factor in a crash that injured 18 soldiers at Sydney's Holsworthy Army Barracks this morning.

The open-top troop carrier rolled in bushland during a training exercise shortly before 9:00am (AEDT) at the barracks in Sydney's south-west.

Two soldiers with severe injuries were airlifted to Liverpool and Westmead hospitals, where one of them is on life-support.

Five people were treated on site and 11 were taken to hospital by ambulance.

Defence says the soldiers' next of kin has been notified.

"One of the injured soldiers has been categorised as critical," it said in a statement.

"Notification of the next of kin of the remaining injured members is underway. Army will not release the personal details of any of the members at this stage."

Police say the 18 Defence Force members who were riding in the vehicle were thrown out as it rolled on a dirt road.

Inspector Steve Blair says paramedics were shocked when they arrived at the crash site this morning.

"With 18 people, a vehicle overturned, [and] people [being] ejected from the rear of the vehicle, it will look somewhat horrific," he said.

"However, the trained personnel that arrived to render first aid and everything are used to seeing people strewn across a large area.

"All the people received various cuts, bruises, abrasions, some with a couple of broken limbs, internal injuries."

'Unusually confronting'

NSW Ambulance spokesman Steve Fulton has described the scene as unusually confronting.

"It was confronting in that there was quite a number of patients," he said.

"Usually in motor vehicle accidents the number of patients tend not to be as high as this one."

Mr Fulton says the site of the accident was well-scrubbed, located some distance into the Army base, making it difficult to access.

"[But] there was nobody that we couldn't account for. Everybody was able to be treated," he said.

He says police, ambulance and Army personnel were all involved in the response.

"All the personnel from each department assisted each other and worked well together," he said.

Mr Fulton said he could not comment on the cause of the incident.