A motorcyclist is dead and his pillion passenger is fighting for life following a collision shortly after they were stopped by police for riding without helmets in Sydney's west.
A critical incident investigation is underway into the circumstances surrounding the crash, which happened just after 1am on Sunday.
An officer tried to speak to the rider after pulling him over on Alpha Street at Blacktown, police say.
The pair were directed to move to the side of the road but as the officer went to do a U-turn, the motorcycle left the scene.
A short time later, the bike collided with a Lexus SUV at the intersection of Bungarribee Road and Balmoral Street.
Police were directed to the scene by PolAir and conducted CPR until paramedics arrived.
Despite their efforts, the 23-year-old rider died at the scene, while his 22-year-old passenger was taken to Westmead Hospital, where he remains in a critical condition.
The four occupants of the SUV were uninjured.
A crime scene was established and examined by specialist officers from the Crash Investigation Unit.