Police actions are being investigated after a man fleeing officers was killed when his car slammed into a school bus in western Sydney.

The silver sedan collided with the bus just after 8am on Friday when it crossed a major intersection in Londonderry "without slowing", Acting Assistant Commissioner Jeff Philippi told reporters.

The driver of the car died at the scene, while an uninjured bus driver was taken to hospital for mandatory testing.

Two of the 10 students on the bus were taken to Nepean Hospital with minor injuries, including head wounds.

Mr Philippi said an unmarked police vehicle "had cause to follow" the car after a traffic violation just moments before the crash.

A critical incident investigation, which is launched when police are involved in a death or serious injury, is underway.

Specialist forensic officers will examine the scene and witnesses are being asked to come forward.

© AAP 2017