Confronting vision has emerged of the moment a WA Police officer was mowed down in a motorcycle hit-and-run incident in Perth’s south.

The officer suffered serious injuries when he was hit by the motorcycle in Warnbro on Wednesday evening.

Vision aired on 7NEWS on Thursday night shows the officer standing in the middle of Holcombe Road trying to hail down the rider before he is struck by the motorcycle, which speeds off without stopping.

The senior constable then slumps to the ground, lying motionless, as witnesses rush to his aid.

State Traffic Commander Mike Bell said the officer was in “good spirits” despite his head and leg injuries, with police still appealing for the rider of the motorcycle to come forward.

“The officer has stepped out on the road to stop the motorcycle,” Commander Bell said.

“The motorcycle has moved further to the right and obviously made his intentions clear that he was not prepared to stop.

“The officer and the motorcycle have come into contact and the officer was knocked to the ground, and the motorcycle continued on the incorrect side of the road and did not stop and render assistance.”

Brian Postans was walking with his partner and dog when the crash unfolded.

He said he heard screams before seeing the bloodied officer lying injured on the road just moments after he was hit.

“(The officer) wasn’t okay, he was definitely in a lot of pain. I went around a roundabout where the incident happened and I could hear him screaming and that’s what made us look over, and then we saw him on the ground. He seemed more shocked than anything,” he said.

Mr Postans and his partner rushed over to help the officer, who was already being treated by an off-duty nurse who passed the scene at the same time.

“He was lying on the ground with his partner, and we basically stopped to go and help because there was only one or two other officers there at the time, and the motorbike had already gone,” he said.

“There was an off duty nurse at the scene as well who wrapped up his head wound and leg wound. It was pretty horrific.”