A five-year-old girl has died after being struck by a vehicle in Sydney's west tonight.

Emergency services were called to reports that two pedestrians had been hit by a Toyota Tarago on Rooty Hill Road North, Plumpton about 7.20pm on Wednesday.

The pedestrians were a 37-year-old woman and her daughter, NSW Police said.

The pair were taken by NSW Ambulance paramedics to Westmead Hospital, with the woman in a serious but stable condition and the child in a critical condition.

The child later died at the hospital.

The 64-year-old male driver was taken to Westmead Hospital for mandatory testing.

He is currently assisting police with inquiries.

Officers from Mount Druitt Police Area Command attended and established a crime scene.

Anyone who may have witnessed this incident or has dash cam footage is urged to contact Crime Stoppers on 1800 333 000.