WA cop cleared of slapping teen at arrest

A policeman involved in the investigation into Corryn Rayney's murder has been found not guilty of twice slapping a handcuffed 16-year-old boy in the face during an arrest.

Sergeant Mark James McKenzie had been on trial in Perth Magistrates Court accused of assaulting the teenager during his arrest in the Pilbara town of Newman in October 2014, but was acquitted by Magistrate Peter Malone on Thursday.

Constable Duncan Cooke testified he saw his colleague give the boy two short, sharp slaps as he sat on the kerb.

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

But Sgt McKenzie's lawyer Darren Jones said there were too many inconsistencies in the evidence of the witnesses.

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

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 admitted "scuffling" with the officers, "mouthing off" and resisting arrest.

Outside court, WA Police Union acting president Brandon Shortland expressed concern about the number of officers being charged as a result of their actions while on duty.

"Unfortunately we've seen a few cases recently where WA police and the state have chosen to prosecute their members without there being a case to answer and we'd like to see that stop," he said.

Sgt McKenzie was the second-in-charge during Ms Rayney's 2007 murder investigation and was criticised by Justice Brian Martin in his reasons for finding her estranged husband, former prominent barrister Lloyd Rayney, not guilty of her murder.

Justice Martin found Sgt McKenzie, who was also the Rayney family's liaison officer, had misrepresented his role to gain information from Mr Rayney.