Two women have been arrested after another woman was found with stab wounds in the boot of a car in the NSW Southern Highlands.

A member of the public called Triple zero this morning, after seeing what appeared to be be a hand waving from the rear tail light section of a white Holden sedan on the Hume Highway at Pheasants Nest.

Later that morning, Highway Patrol Officers spotted the vehicle on the Hume Highway at Berrima.

The two other women, aged 24 and 18 were arrested and Conveyed to Southern Highland Police Station.

An investigation is underway.