A senior WA police officer involved in an incident in which a male pedestrian was struck and knocked down by a police car during a dramatic chase has been stood down.

Police said the senior sergeant was stood down on Thursday afternoon following the emergence of dramatic new footage showing the man convulsing on the ground.

An internal police investigation is underway into the actions of police involved in the incident.

The previously unseen video was shot by a passer-by, Chelsea, who said she could not believe what she had seen.

"When we saw the white four-wheel-drive pull out and swerve into him we were just shocked," she said.

A screenshot showing a police car hitting a man in Thornlie, prompting a police investigation. ( Supplied )

A man is thrown to the ground after being hit by a police car in Thornlie. ( Supplied )

"Then I just couldn't get my head around why he actually swerved the car into him, why the lady officer didn't grab him, or why they didn't pepper-spray him."

"Then, after the video stops, we saw the boy get handcuffed while he was having a fit."

The 18-year-old had been walking along Wilfred Road in Thornlie when he came to the attention of police following a caller complaint of suspicious activity in the area.

Police said he was located but a scuffle ensued in which an officer was injured, before the man fled on foot and was arrested a short time later after he was hit by the police car.

The man, who was not charged, was taken to Fiona Stanley Hospital and treated for injuries. He now awaits the results of a CT scan.

The actions of the officers involved are subject to an internal police investigation.

The man reportedly sustained bruising in the incident. ( Supplied: Mervyn Eades )

'I don't want my kids targeted'

Chelsea, who did not want her surname published, said the incident made her fear for her own teenage children.

"It makes me feel angry and it makes me feel worried for my children when they grow up, being that they're Aboriginal children," she said.

"I don't want them to be targeted by the police because they're in the wrong place at the wrong time."

Family spokesman Mervyn Eades said the video showed the disturbing extent of the impact.

Mervyn Eades from the Deaths in Custody Watch Committee said the actions of the police involved were excessive. ( ABC News: Nicolas Perpitch )

"What it did show me, that was very disturbing for me and the family, was moving him after he'd been hit by a motor vehicle and started convulsing."

"He could have hit his head on the kerb and it could have been fatal, to think that a police officer could do that, it shows no duty of care at all."

Mr Eades said he was hopeful the investigation by police was full and thorough.

"We are hoping he is stood down without pay and, if need be, criminal charges be put against him because that is dangerous or reckless driving," Mr Eades said.

"It needs to send a clear message out to the force for those rogue officers that wish to do this sort of stuff, that it is not acceptable."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 30 seconds 30 s Man struck by police car

Mr Eades, who is also part of the Deaths in Custody Watch Committee, said he feared incidents of this nature were more widespread.

"How many people have been injured which haven't been captured on video?" he asked.

Opposition police spokesman Peter Katsambanis had not yet seen the latest vision uncovered by the ABC but said regardless, the incident highlighted a need for WA Police to use more body cameras and dash cameras.

"It's confronting vision that we saw but there is an internal investigation going on," he said.

"What we're seeing is a proliferation of third-party videos, what we don't have is comprehensive body cameras on our police or car cameras that can record the whole incident.

"We've got the budget coming up today and I hope and call on the Government to fund body cameras for police so that we have the vision that is needed."