A man has suffered a severe leg injury after being hit by a car in Sydney's west.

The man, believed to be aged in his 40s, was reportedly on the footpath when he was struck by a car on Mary Street in Auburn just before 10am.

The man's leg was "partially severed", an ambulance spokesman said, and the man was treated at the scene before being rushed to Westmead Hospital in a serious but stable condition.

The female driver of the white sedan, believed to be aged in her 60s, was assisting police with their inquiries.

The woman was assessed by paramedics after complaining of chest pain, and taken to Auburn Hospital.