Two boys, aged 16, have been charged over what's believed to be a drug deal gone wrong, which police said left two men critically injured in Karrinyup, in the city's north.
CCTV captured the moment before the shocking incident yesterday afternoon when two allegedly armed teens approached a parked ute with two 18-year-old men inside.
South African former rugby player Brynard Stander said one of the teenagers was holding what appeared to be a large kitchen knife.
Police alleged the boys threatened the men, leaving one with a stab wound to his stomach and the other with slash wounds to his left wrist and thigh.
Both alleged victims were rushed to the Royal Perth Hospital with critical injuries but later improved to a stable condition.
One of the teens allegedly grabbed a bag containing cannabis from the ute before fleeing with his alleged accomplice down an alleyway between Northgate Street and Edmondson Crescent.
Stander got out of his vehicle and gave chase.
"I approached him, and I said, 'I saw you had a knife, and I've also got cameras at the front of my house, so you should stop, I'm going to call the police'," he said.
The teen couldn't outrun the former flanker.
"We were running, I said to him, 'Mate, if you want to keep running, I can run quite far, so I suggest you just stop and wait for the police'," Stander said.
Officers who arrived shortly after arrested one of the boys on Jackson Avenue and found the other hiding in a bush, as the injured men were treated.
One of the 16-year-olds, who was also injured, was treated at Perth Children's Hospital before being released into police custody.
Both teens were charged with grievous bodily harm and one of them was also accused of aggravated armed robbery.
Premier Roger Cook called the incident a "horrific crime".
They were called before Perth Children's Court today but couldn't appear by video link due to a technical issue.
Instead, they were remanded in custody and will face court next Monday.
