A boy has saved the life of a woman who was trapped in her car for three days after rolling it down an embankment on Christmas Day.

Police say the 44-year-old woman was driving home after a visit to family when she veered to avoid a kangaroo at about 3:00pm (AEDT) on Wondalga Road, near Batlow, in southern New South Wales.

The car careered down an embankment and flipped onto its roof, pinning the woman by her leg.

But emergency services were only called when a teenage boy walking to a nearby farm heard the woman's cries for help yesterday evening.

Acting Sergeant Roy Elmes says the car could not be seen from the road.

"It's quite astounding and she's very very lucky. I'm sure she would have been quite relieved to have someone just happen to walk past and she's had someone to call out to and the rescue's been not too far away after that," Sergeant Elmes said.

"She does have some serious injuries. She's lucky though. I think the weather's been a bit cooler and we've had some rain, otherwise I think it could've been different circumstances."

The woman was flown by helicopter to Canberra Hospital, where surgeons had to amputate her leg.

She is listed in a stable condition.

Inspector Eamonn Purcell from the New South Wales Ambulance Service says the woman's leg was partially severed when paramedics arrived at the crash site.

"The patient had a severe lower leg injury and was trapped by the car body, with the full weight of the car on her leg," he said.

Paramedics believe the car's weight acted as a tourniquet to stop the woman bleeding from her leg.