Police are guarding a man in hospital after he allegedly ran a red light, fatally struck a woman in another car and drove off in Sydney's south-west on Sunday.

Crash investigators allege the 20-year-old driver smashed into a Toyota Yaris at Prestons about 9pm.

A woman, 57, was taken to Liverpool Hospital but was pronounced dead on arrival.

Her husband, 55, has been treated for shock.

Police Inspector Adam Powderly said it was devastating to see a family ripped apart just days before Christmas