A motorist was punched and slashed with a knife by the driver of another vehicle in an apparent road rage attack in Sydney's south, police say.

The two men were believed to be involved in a three-car crash on the Princes Highway in Kirrawee just before 11pm when one of the drivers turned violent.

The injured 39-year-old man told police he was driving his black Holden Commodore along the Princes Highway when the car in front of him slowed suddenly, causing him to brake.

As a result, the car following the Commodore crashed into the back of the vehicle, shunting it into the vehicle ahead.

Police said a man, believed to be the driver of the first vehicle, opened the Commodore's door and began attacking the driver.

The man was punched in the face and slashed with a knife. A woman who was sitting in the passenger's seat at the time was not injured.

The 39-year-old man initially believed he was not seriously injured, but later took himself to Sutherland Hospital when he discovered his chest had been slashed.

Hospital staff called police, who are appealing for anyone who saw the crash to contact Crime Stoppers on 1800 333 000.