In scenes that terrified onlookers, a mentally disturbed truckjacker took his own life after stabbing a Samaritan and commandeering a cab during a four-suburb rampage through Sydney.

The drama began when police spotted the 24-year-old behind the wheel of a 4WD without plates and attempted to make him pull over.

Rather than obey he rammed and disabled the police car before abandoning his  own vehicle in Chullora, where he took control of a Woolworths delivery van and embarked on an odyssey of destruction, ramming and sideswiping a number of other vehicles.

The man, who was known to police, was next involved in a four-vehicle smash in Rockdale, where he stabbed a gym patron who came outside to render assistance after what he appears to have believed was an innocent fender bender.

The bystander who was attacked at Rockdale is in his early 20s, authorities say.

He was reported in a stable condition late on Friday night at St George Hospital after undergoing surgery.

A woman whose car was also hit by the stolen truck suffered minor injuries and was treated at the same hospital.

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Video of the confrontation shows the injured man was attacked in the middle of the road at Rockdale.

A witness told AAP his friend saw the attack at close quarters and reported seeing the victim stabbed in the stomach with what appeared to be a large kitchen knife.

The injured man clutched his bleeding abdomen and ran to the side of the road while screaming “Call the police! Call the police!”

The offender, who had a history of mental health issues, subsequently carjacked a taxi and drove north to Arncliffe, where he was confronted and surrounded by police outside St Francis Xavier’s Primary School.

“The offender then stabbed himself to the chest and as a result is now deceased,” Acting Assistant Commissioner Mick Fitzgerald told reporters.

“The offender wasn’t trying to intentionally harm people on the way. He could have, if he wished, harmed a great number of both humans and vehicles.”

Police said there was no indication the incident was terror-related.

Henk Wellsare, from Arncliffe, said he had been home for about 15 minutes when he heard a helicopter hovering above his house.

He walked outside to see about 10 police cars and a SWAT team 100 metres down Forest Road.

“As you got closer you could see someone lying on the ground, people standing around him,” he said.

– with wires