A MAN in his 20s stabbed himself to death after a terrifying crime spree across Sydney that included another stabbing, a carjacking and several crashes.

The incident unfolded when police tried to stop the 24-year-old driver of a 4WD because he was not displaying number plates on the Hume Highway, at Bass Hill in Sydney's west, just before 3pm yesterday.

Police said the man rammed one of their vehicles as he escaped and drove east towards Chullora, where he dumped the 4WD and stole a supermarket delivery truck from Waterloo Road.

The driver from Greenacre collided with up to five cars in Wiley Park, southwest Sydney, before smashing into several more on West Botany St in nearby Rockdale. He then ditched the stolen truck and started fighting a man who tried to intervene after seeing the commotion outside Fitness First gym.

Vision stills show the moment a man car-jacks a cab outside Fitness First in Sydney's Rockdale. The attacker later took his own life.
Vision stills show the moment a man car-jacks a cab outside Fitness First in Sydney's Rockdale. The attacker later took his own life. Abbas Salim

Acting Assistant Commissioner Mick Fitzgerald told reporters the bystander was "attempting to stop the offender from leaving the scene" when he was stabbed.

"(The attacker) has a mental health background. He has not got an extensive criminal history, but he is known to police," Acting Ass. Commissioner Fitzgerald said.

"It was a man doing everything he could to avoid arrest. His actions, particularly when he stabbed an innocent person, are the actions of someone we believe who had mental health issues.

"It would appear to us that the offender wasn't trying to intentionally harm people along the way. He could have, if he wished to, harm a great deal of (people)."

Chris Burton, a tradesman who witnessed the Rockdale stabbing and went to the victim's aid.

"He was laying there, like, 'I'm going numb'. It was a good knife. It went all the way in," Mr Burton told 10 News.

"I put my hand on (the wound), sat him down and told him what my mother taught me, breathing techniques."


The Woolworths van at the stabbing scene in Rockdale.
The Woolworths van at the stabbing scene in Rockdale. Picture: Instagram user @color_me_beautiful_by_jain

One witness described the attacker "was looking crazy and frazzled" before he carjacked a red taxi and fled the scene.

The incident was captured on video by witnesses and shows the two men fighting in the middle of the street before the victim is stabbed. Another video shows the attacker carjacking the taxi moments later. A Rockdale resident named Con described the attack as chaotic.

"There was a bloke in a Woolworths van and then he side swiped two cars there and side swiped another two cars, then he got out of the car and had a knife," he told The Daily Telegraph. "He was trying to hijack other cars, he's gone across the road and a kid from the gym - he's stabbed the kid then hijacked a cab and left," he said.

The victim has been taken to St George Hospital in a serious condition. A hospital spokesperson told news.com.au at 6pm the man's condition had improved and he was stable. The man was understood to be undergoing surgery on Friday afternoon.

Acting Ass. Commissioner Fitzgerald praised those who ran towards danger to help the "good Samaritan" who was stabbed.

"Indeed, the members of the public who came to the aid of other drivers and the victim in West Botany Street are to be commended and will receive formal commendation from police," he said.

It would have been an horrific scene for people to witness, he said.

A related incident unfolded on Forest Road, near St Francis Xavier's Catholic Primary School in Arncliffe, soon after the attacker fled Rockdale.

Images of a body covered in a white sheet on a road in the suburb started to appear on social media before police confirmed the offender had died after turning the knife on himself.

Police and ambulance services had multiple crews on scene and there were reports of helicopters in the area. A NSW Police spokesman said there was no indication the matter was terror-related.

"We are fully complying with the police investigation and are unable to provide further details," the spokesperson said.

"Our priority is ensuring our staff and members are provided support at this time."

Both scenes are taped off and police are interviewing witnesses.

If you or someone you know needs help, contact Lifeline on 13 11 14 or visit lifeline.org.au