Sydney carjacking, stabbing and motorway crash after police pursuit

There have been scenes of mayhem across Sydney's west, with a carjacking, collision and stabbing after a motorist drove into oncoming traffic on a busy motorway with two children on board.

Police were called to carry out a welfare check on a 36-year-old driver in Belmore, but began a pursuit after he refused to pull over.

"When the driver allegedly failed to stop as directed, a police pursuit was initiated," NSW Police said in a statement.

"The pursuit continued into Auburn before it was terminated due to the manner of the man's driving."

Police said the man then drove on the wrong side of the M4 Motorway before hitting an oncoming vehicle.

He then got out of the car with the two children and allegedly tried to steal a car.

A 25-year-old man intervened and during a scuffle suffered a stab wound to his hand.

The offender, police allege, then stole the vehicle of a woman who had stopped to help before bundling the two children into her car and speeding off westbound along the M4 Motorway.

He stopped on the Cumberland Highway at Wentworthville and was arrested by an off-duty police officer with help from bystanders.

He was taken to hospital under police guard.

The children suffered minor injuries and were taken to Westmead Children's Hospital as a precaution.

Police are still waiting to interview the man as they piece together what has happened.