Police are appealing for witnesses to a fatal crash on the M4 motorway near Homebush in Sydney's west.

Around midday on Friday, a 30-year-old man was killed when his motorcycle hit a concrete barrier, throwing him from his bike.

He was then struck by a car and run over by a ute.

He died at the scene.

Police say they took witness statements from drivers when the crash occurred.

But they want to speak to the driver of a black car thought to be travelling east at the time, who they believe may have more information.