Emergency services were called to Hassans Walls Road at 4.45pm on Tuesday following reports of a car skidding off the road and crashing.

Lithgow Police Station Sergeant Darryl Goodwin said a silver Ford Focus was descending from Hassans Walls Lookout on the road towards Lithgow when the crash occurred.

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"There was a male driver and two female passengers," he said.

"He was on his red Ps and came around a corner and lost control on a dirt road ... it's fishtailed and it's gone off the edge.

"It's collided with a number of trees and then come to rest against a tree 10 metres down [the hill]."

Sergeant Goodwin said the 17-year-old male driver and 16-year-old rear passenger escaped uninjured, but the 16-year-old front seat female passenger had a severely broken upper left arm with a compound fracture.

"She lost a lot of blood," he said.

"First aid was rendered at the scene by the driver and passers by."

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Police, fire and ambulance crews were called to the scene and the injured passenger was stabilised on the side of the road by paramedics.

"They put her in a stretcher and carried her out," Sergeant Goodwin said.

"She was airlifted to Westmead Hospital where they performed surgery on her last night."

The teenager was in a serious but stable condition when she was airlifted from the scene.

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The male driver was taken to Lithgow Hospital for mandatory blood and urine testing.

Crash investigators were called to the scene and were on site for four hours.

"The crash is still under investigation and we're waiting on testing to come back," Sergeant Goodwin said.

Early indications are that the driver's speed could have been a factor.

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