The young Aboriginal girl who was pinned to the floor and thrown into a train seat by a WA Police officer has spoken out.

On Friday, 'Sarah' (not her real name) told National Indigenous Times she had a lump on her head and pain in her back and neck days after the incident on Sunday.

She said she and her friends had been harassed by a group of boys earlier on the train who then assaulted her niece and stole a necklace from her friend.

"I got up and said to those boys 'how shame are you, you have to go up to people and steal chains right off their neck?... what is wrong with you?'," she said.

"I reached out to grab the chain because it was within reach and he pulled it back ... that was when that police officer pushed me back, and when he pushed me back he touched my boob, my chest, I felt uncomfortable."

The officer was off duty, out of uniform and travelling with members of his family at the time.

Sarah said she "rotated" her hand up to get away from the officer, making contact with his face, before turning her back. She said the officer then accused her niece of striking him in the face, so she turned around and took responsibility for it.

"I said 'I am sorry for that, I really apologise for that', and that's when I got tackled," she said.

It was then that her niece took her phone out and started filming.

"I was already on the ground before she could get her phone out," Sarah said.

"I hit my head on the corner, on the side of the chair. He was kept trying to touch my shoulder, trying to lift me up, I said 'don't touch me' and kept moving his hands away.

"When he lifted me up, he full on lifted my bra strap - I'm going to be straight up, my boob fell out of my bra, then I got thrown. And during all of that I had a panic attack. I didn't even know what to do."

The video shows the officer then pinning Sarah in the train seat with his knee.

"He had his knee in my private parts," she said.

"He got off me when they pulled up to the (Perth train) station.

"They kept me under arrest at the station; police officers came to the station and questioned me.

"They drove me to Perth police station - the watch house - they kept me in the cell for about 30 minutes then my aunty came and bailed me out."

Sarah was issued with a juvenile caution and said the officers "didn't really say anything else".

The 13-year-old said she the officer should be investigated and dismissed.

"If he wants to be racist, I want him to lose his job," she said.

A spokesperson for WA Police confirmed Sarah had been charged with common assault and issued a juvenile caution, and said the incident is being reviewed.

"Relevant evidentiary material, including Public Transport Authority CCTV, has been obtained and is being reviewed by the WA Police Force as part of standard process," the spokesperson told National Indigenous Times.

WA Minister for Police, Reece Whitby, told National Indigenous Times: "We expect all WA Police officers to always act with professionalism and appropriate judgement."

"This matter is under review as is standard practice, including internal processes and CCTV, and it's important those inquiries run their course," Mr Whitby said.

Note: Sarah advised National Indigenous Times she had consent from her parents and grandmother to speak to this publication.