Steve ran to the scene of a car crash to help. He saw a woman being attacked by a knife-wielding man.

Brave samaritans have been praised following a stabbing rampage on a major Sydney highway that left a woman fighting for life and three others, including the alleged perpetrator, wounded.

Two cars collided about 9am in Engadine in southern Sydney on Sunday before passers-by rushed to free the distressed female passenger who was screaming for help.

Police believe the driver, aged 57 or 58, began stabbing his partner while the car was weaving through traffic quite erratically before the crash.

A 58-year-old man has allegedly gone on a stabbing rampage at Engadine in Sydney's south after a crash on the Princes Highway.

A 28-year-old police officer who got stabbed while making the arrest has now been discharged from hospital. #9News

DETAILS: https://t.co/frCRezfk25 pic.twitter.com/O7XVfiuXSr

— 9News Sydney (@9NewsSyd) August 25, 2024

The man was known to them but not for domestic violence matters involving the woman, police said.

Any other domestic violence links would be explored, they said.

Local commander Donald Faulds credited the "amazing work" of those who intervened.

"They've just turned up thinking it is just a normal motor vehicle accident ... then they're faced with a box cutter," Superintendent Faulds told reporters.

"Quite frightening but very, very brave as well."

Images from the scene showed blood splattered across the inside of the front passenger's door, seat and across the white sedan's paintwork.

Cronulla Sharks NRL chairman Steve Mace said he came face-to-face with the knife-wielding man after the crash happened right in front of him on his way to his young son's junior football game.

He ran to the passenger's aid after hearing the 49-year-old's screams but struggled to open the door of the sedan to save her.

"I ... finally jammed it open and all of a sudden, I see him just slashing her," he said in a video posted by the Daily Telegraph.

"(He was) just cutting her neck, her breast ... in the sides, wherever he could go."

Mace described how another man, who was also trying to get the woman out of the car, ended up with a gash from his shoulder to his belly button before the alleged knifeman left the car and made repeated attempts to attack the group.

"It was like a stand-off, which felt like an hour," Mace said.

"(He was going on about) it's a big conspiracy about the jab, and we're all getting followed, and we're all dead anyway."

There was also self-harming and an alleged attempt to flee after police arrived and tasered him, and an officer suffered a slice wound to his wrist during the arrest.

The driver and passenger were taken to hospital in a critical condition, though the man had stabilised by 2pm.

The injured Samaritan was in a serious but stable condition in hospital.

Two people in the other car were also treated for injuries from the crash.

Mace said he escaped with just a tiny cut to his finger.

Witnesses were urged to provide any footage surrounding the time of the incident to Crime Stoppers or local police.

A five-kilometre stretch of the Princes Highway was closed in both directions.

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Feature image: AAP