Police have arrested one of two men allegedly responsible for a fatal hit-and-run while driving a stolen car in Sydney's south west.

A 70-year-old man was walking along a street at Ambarvale this morning when he was hit by the speeding car that then overturned and ended up on the front lawn of a property.

The victim died a short time later in hospital.

Detectives are questioning a 22-year old man who was arrested this afternoon in the Bradbury area.

Officers are still searching for the second man who also allegedly ran from the car after it overturned.