Police have launched a critical incident investigation after a man's violent crime spree across several suburbs in Sydney's south, which ended when he killed himself.

Key points: Police said the offender killed himself after stabbing another man and stealing two vehicles

Police said the offender killed himself after stabbing another man and stealing two vehicles The Greenacre man, 24, had mental health issues according to police

The Greenacre man, 24, had mental health issues according to police Bystanders who helped victims at Rockdale will be formally commended

Police say the spree began about 3:00pm when officers tried to stop a 4WD on the Hume Highway at Bass Hill, because it was not displaying number plates.

The 4WD rammed police cars and drove towards nearby Chullora, where the 24-year-old driver dumped the vehicle and stole a supermarket delivery truck from Waterloo Road.

Police say as the truck headed south towards King Georges Road, it hit several vehicles along the way, before becoming involved in a multi-car crash on West Botany Street in Rockdale about 3:10pm.

The Rockdale stabbing occurred in the middle of the road as onlookers watched on in horror. ( Facebook: Ahmed Jay )

It was there that onlookers watched in horror as the driver stabbed a good Samaritan in the abdomen as he attempted to prevent the man from stealing another vehicle.

The victim was heard shouting "call an ambulance" as bystanders shouted and screamed in shock.

The victim, who is in his 20s, was taken to St George Hospital where he is undergoing surgery, police say.

Map Map shows how the crime spree unfolded

The man managed to force a taxi driver out of his vehicle before stealing it and driving it to Forest Road in nearby Arncliffe.

About 3:30pm he got out of the taxi and was confronted by police.

He then killed himself in front of them.

NSW Police said the offender was a 24-year-old man from Greenacre, who is believed to have had mental health problems but did not have an extensive criminal history.

Bloodied clothes are scattered along the side of West Botany Street in Rockdale after the stabbing. ( AAP: Heather McNab )

Extra police were called to the scene at Arncliffe on Friday night after the man's family began arriving.

NSW Police Superintendent Michael Fitzgerald said he did not believe the man was trying to intentionally harm people, given the extent of the crime spree.

"Obviously he … has hurt people, but he could have if he wished to, harm a great deal of both humans and vehicles if he wished to," he said.

"It was a man who was attempting to do everything he could to avoid arrest."

Sorry, this video has expired Acting Assistant Commissioner Michael Fitzgerald says police confronted the offender before he took his own life.

Superintendent Fitzgerald said the entire spree was "horrific" for members of the public to witness.

"His actions — in particular the actions when he stabbed the innocent person — are the actions of a person who we believe had mental health problems," he said.

Superintendent Fitzgerald praised bystanders who rendered first aid to the man stabbed at Rockdale and said they would receive a formal commendation from police.

A critical incident team will investigate the circumstances of the incident and the investigation will be independently reviewed.