A WA Police senior sergeant has been stood down after new footage emerged of an incident in which a man was struck by a police vehicle in Thornlie.

The 18-year-old was hit on Sunday and required treatment in Fiona Stanley Hospital.

Police said they were responding to a call about suspicious activity when they located the man and a scuffle ensued.

They say the man fled on foot and was arrested on nearby Wilfred Road about 3pm after being struck by a police

The incident and the actions of police are being investigated by the Internal Affairs Unit.

The officer has been stood down from all operational duty pending the outcome of the investigation.

Witnesses to the incident are asked to contact police on 131 444.