"Police takes all reports of criminal offending seriously and will follow up as appropriate," said a spokeswoman.
"We ask that people please contact us if they witness a crime taking place, rather than taking matters into their own hands."
The shocking clip, which lasts less than a minute, takes place inside a moving vehicle as the driver apparently films an interrogation over what he says is an attempt to steal his car.
The passenger, who the driver accuses of stealing the vehicle, denies it repeatedly in the video, saying he was trying to fix the car.
"No, it wasn't going or anything, bro. It had no oil in the radiator," the passenger says.
The driver presses him, asking him what he means.
"Like, I was trying to fix the chassis and that for yous [sic]," he replies.
"Chassis? How do you know to fix my own car bro? There's nothing wrong with it."
The passenger keeps trying to justify tinkering with the car when the driver suddenly lashes out, striking the teen forcefully across the mouth.
Recoiling from the blow, the passenger is asked again if there was anything wrong with the car.
His hand held over his mouth, he says there was nothing wrong with it.
The driver then threatens the passenger with more violence and warns him to keep away from his neighbourhood.
The passenger is brought to tears, cowering in his seat and telling the driver he is sorry and didn't mean to mess with his car.
The video has been credited on Facebook to a poster who appears to have links to the Mongrel Mob.
Police said this morning it was difficult to comment about the video and alleged assault without specific details of when and where it happened, or who was involved.
