WA cop saw colleague slap teenager twice

A policeman saw his colleague twice slap a handcuffed 16-year-old boy in the face with an open palm when the teen continued to swear and yell, a Perth court has heard.

Constable Duncan Cooke testified on Wednesday the boy was being boisterous when police were called to a Halloween party in the Pilbara town of Newman that had begun to disperse in October 2014.

The defendant, Sergeant Mark James McKenzie, spoke with the teenager and the officers attempted to arrest him, but he resisted until Const Cooke managed to handcuff him and he sat down on the kerb.

The boy continued to be "verbally aggressive" towards McKenzie, who then gave the boy two short, sharp slaps, Const Cooke told the Perth Magistrates Court.

He said he was shocked by the assault and noticed the boy's demeanour changed - he stopped yelling and had tears in his eyes.

"He became very submissive and childlike in a way," Const Cooke said.

He told the boy afterwards if he felt he had been treated unfairly, he could complain.

"I felt that he had been treated unfairly," Const Cooke said.

The officer said he asked another sergeant for advice about two days after the incident and was later called in to speak to the officer in charge of the station about it.

Const Cooke said he believed he had a good working relationship with McKenzie before the assault.

"I've got a lot of respect for Sergeant McKenzie ... still do," he said.

Under cross examination, Const Cooke was asked if it was possible McKenzie had been putting the boy's beanie over his mouth. He replied: "I don't think it was. I think he slapped him."

The boy also testified his face was swollen and red from the assault and he also had marks on his wrists from the handcuffs.

He said he was unaware he could do anything about it until officers told him he could.

"Coppers, they do what they want to do," he said.

The boy said he had several alcoholic drinks that night and admitted "scuffling" with the officers, "mouthing off" and resisting arrest, but he did not think that excused any assault.

A woman filmed some of the arrest and testified McKenzie's behaviour was inappropriate but she did not see him slap the boy.

Const Cooke also viewed the footage and said the slaps happened during the filming but were not captured on camera.

The trial continues.