Thief pulls knife on WA Labor leader's son

A bike thief pulled a knife on the West Australian opposition leader's son and threatened to kill him after the 13-year-old gave chase in Perth's south on the weekend.

Mark McGowan told reporters his two sons saw the thieves taking their bikes as they emerged from their grandparents' house, a few kilometres away from the family home in Perth's southern suburbs, on Saturday.

"My eldest boy pursued one of other boys for a few kilometres on foot and he eventually caught up with him because the chain fell off the bike, and the other boy pulled a knife out of his backpack and threatened to kill my son," Mr McGowan said on Monday.

"My son backed off at that point in time, which I'm very pleased about. The other boy ran off and he gave up the chase.

"He didn't know that was going to happen - he was doing what he thought was right."

Mr McGowan said the incident was very disturbing but his son wasn't particularly shaken up afterwards.

"I think maybe the adrenaline was still pumping," he said.

"I told him 'you were very brave, but you've got to be very careful'.

"I'd counselled the boys before they went about safety on the roads, make sure their helmets were on, all the things you worry about. But then it turned out when they got there, it wasn't a road safety issue that confronted them."

The boy who brandished the knife was aged about 14, Mr McGowan's son estimated, and had been carrying it in his backpack.

The incident was reported to police who attended the McGowan residence to get statements.