A man took his own life after carjacking two vehicles and knifed a bystander in a rampage through four Sydney suburbs.

The chaos began when police tried to pull him over on the Hume Highway in Bass Hill about 3pm because he wasn't displaying number plates.

Instead of stopping, the 24-year-old rammed a police car and kept driving before ditching it in Chullora and stealing a Woolworths delivery truck.

A knifeman took his own life after carjacking two vehicles and stabbing a bystander in a rampage through four Sydney suburbs

The man, who had mental health problems, lay dead on the road near St Francis Xavier's Catholic Primary School

The truck drove along several other roads, sideswiping at least four other cars along the way before crashing into several cars in Rockdale.

He got out of the truck near a Fitness First gym, the same one where mobster Mick Hawi was shot dead last February, and began abusing people in Arabic.

A 20-year-old man came out of the gym and tried to calm the driver down, but was instead stabbed in the stomach with a kitchen knife, witnesses said.

The assailant then carjacked a red taxi and fled the scene, with video posted to social media showing him take off on the wrong side of the road and mount a kerb.

Police caught up to him on Forest Road in Arncliffe about 3.30pm, near St Francis Xavier's Catholic Primary School.

The moment a 20-year-old man came out of the gym and tried to calm the driver down, but was instead stabbed in the stomach with a kitchen knife

The assailant then carjacked a red taxi and fled the scene, with video posted to social media showing him take off on the wrong side of the road and mount a kerb

Officers gave him first aid but he died at the scene. A white sheet was seen placed over the body

He got out of the car and took his own life as police looked on in horror.

Officers gave him first aid but he died at the scene. A white sheet was seen placed over the body.

The knifeman was known to police and had a history of mental health issues, police said.

The young man who was stabbed outside the gym was in a serious but stable undergoing surgery in St George Hospital.

NSW Police Acting Assistant Commissioner Mick Fitzgerald said the man had a 'mental health background' but not an extensive criminal history.

'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,' he said.

'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 both humans and vehicles.'

The Woolworths truck by the side of the road after it crashed into several other cars and stopped, before the driver got out, stabbed a bystander, and carjacked a taxi

The red taxi at the scene in Arncliffe after the knifeman took his own life in front of police

Officers and paramedics examine the body of the man after he took his own life in front of them

Chris Burton witnessed the Rockdale stabbing and went to the victim's aid as he lay on the gorund in a pool of blood.

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

'I put my hand on [the wound], sat him down and told him what my mother taught me, breathing techniques.'

NSW Ambulance Inspector Jordan Emery said bystanders like Mr Burton and others and who ran to his aid helped save the victim's life.

'We arrived on scene to find a truck had collided with two vehicles and there was a male patient who had sustained an abdominal stab wound,' he said.

'The injured male had sustained a significant stab wound to his abdomen and was lying on the roadside with a number of members of the public and police providing first aid to him.'

West Botany Street remains taped off and police are interviewing people at the scene

The man got out of the car and took his own life as police looked on in horror

Mr Emery thanked members of the public and police for their quick-thinking.

'It was quite a dynamic scene, there was a number of bystanders crowding around the patient, police did a tremendous job providing first aid to this male and it makes all the difference in these time critical emergencies,' he said.

'Members of the public did provide direct pressure to the wound which is one of the hallmark treatments for life-threatening haemorrhage. They provided that care to him prior to police arrival.'

Acting Assistant Commissioner Fitzgerald also praised the people who ran to help the injured man in a dangerous situation.

'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.

A female driver who was in one of the cars hit by the Woolworths truck suffered minor injuries and taken to St George Hospital.

West Botany Street remains taped off and police are interviewing people at the scene.

LIFELINE: 13 11 14 www.lifeline.org.au or MensLine Australia - 1300 78 99 78