Isaiah Barney, who filmed the incident in Kununurra earlier this month, told AAP a woman was being restrained by two policemen when one officer struck her in the head in front of a stunned group of onlookers.
"One officer was on her legs and the other officer was on her back and shoulders so hard," Mr Barney said.
"I was looking at her - it was hard for her to breathe - and then I started recording.
"I was absolutely shocked, I had to literally re-watch the video to see him hit her, but I had it on camera."
Mr Barney said police had broken up a confrontation between his sister and another woman in the Kimberley town on the early hours of December 5.
"I immediately thought they had my sister and so I ran towards them and found they had the other woman on the ground," he said.
"I made it very clear to the police officer in that video that I was recording him, I screamed it out to him ... he struck the lady two times in the head."
Police have been contacted for comment.