The officer driving the unmarked car at the centre of the disturbing footage is under investigation.

A West Australian police officer at the centre of shocking footage showing a police car hit an Indigenous teenager has been stood down from active duty after NITV News revealed additional footage of the incident yesterday.

The footage shows West Australian Police hit 18-year-old William Farmer with an unmarked police car, and appears to show the police car crossing the road and cutting him off before knocking the man to the ground.

In a statement provided to NITV News WA Police media said the matter was being investigated internally by police.

"The officer, a senior sergeant, was stood down from all operational duty pending the outcome of the investigation by the Internal Affairs Unit," the statement said.

George Newhouse, principal solicitor of the National Justice Project, is providing legal support for the man's family.

“The family want to see a very thorough investigation undertaken, and that appropriate measures are taken to ensure that this never happens again, and if it’s appropriate that criminal charges be laid,” he told NITV News.

“What I find particularly concerning about that image, is that as William is having a fit, instead of showing compassion and providing assistance, it appears that he's handcuffed. And that seems totally inappropriate and in fact cruel.

"He’s never fitted before and the family are incredibly distressed and concerned about his situation, his physical injuries and they’re currently investigating those and we don’t know the results as yet," Mr Newhouse said.

"He was an Aboriginal man simply walking down the street and being followed by a police officer. There was no need to apprehend him in this violent way, and we’re totally at a loss to understand how or why this incident took place and if it wasn’t on film I’m not so sure that you’d be seeing the intense investigation that’s currently underway."

Earlier this week, police claimed the incident occurred when the teenager “collided” with the police car after another video initially emerged in which the impact was not visible. The new video which was shot from the other side of the car and up close appears to show a different story as the car clearly drives across the road following the teenager before driving into his path, knocking him with such force that he ends up on the footpath.

Mr Newhouse said the teenager has been going back and forth between hospital and home for tests since the incident.

The teenager is awaiting the result of tests after the incident left him on the side of the road in convulsions.

Source: Supplied

Australian Greens Senator Rachel Siewert called for an independent investigation into the incident.

“The second tranche of footage that shows WA police veering into the path of an Aboriginal man and knocking him on to the footpath is extremely distressing.

“I believe this is indicative of a broader problem within the WA police in regards to some officers' attitude and treatment of our First Peoples and should be taken very seriously.

Gerry Georgatos from the federal government's National Indigenous Critical Response Service told NITV News the collision was "deliberate".

"The individual was purposely knocked over by the police vehicle. Then he appears in a fit like state and instead he is maltreated further, flipped around while in a fit like state and handcuffed," he said.

"The way he was tossed was reminiscent of the abominable treatment of Ms Dhu. Internal Affairs has to do its job to clean up this type of brutal behaviour."

CEO of Ngalla Maya Aboriginal Corporation Mervyn Eades, who was given the footage by the couple who filmed it, said the incident showed there were systemic issues in the way WA Police engage with Indigenous Australians.

"My people experience this type of confronting heartlessness again and again, it is endemic," we told NITV News.

"For every incident captured on footage we've an encyclopaedia long list of incidences told to us direct and through the grapevine. Police have to get their act together."

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