A man who crash-tackled a teenager minutes after he allegedly killed two pedestrians with a stolen car said he had no idea of the terrible tragedy that had unfolded streets away.

Kingston Ave resident Adam Lauriston said he spotted a teenager in his courtyard around 5.30pm.

"I had no idea what had happened," he said.

"It wasn't until after I was walking back that my neighbour grabbed me and (told me two people had been killed)."

Mr Lauriston said the teenage boy had been in his courtyard and ran down Kingston Ave.

"A neighbour came out and pulled me off of him and then cops arrived about 10 minutes later."

Mr Lauriston and a neighbour detained the teenager until police arrived.

"I stood there and just explained to him [not to leave] … so he just sat there and didn't move anymore really," he said.

He said the teenager appeared confused.

Mr Lauriston said when police arrived, they described the nearby crash scene as "the worst thing we've seen in a long time".

"At the end of the day, it's up to the court to do their job now."

Originally published as What happened when teen driver was found