WA Police have launched an internal inquiry after a magistrate ruled a senior police officer had used violent, unreasonable force against a 13-year-old boy during an unlawful arrest.

In a scathing judgment, magistrate Michelle Ridley described the actions of Sen. Const. Grant George as “forceful”, “aggressive” and “harmful”.

She said the fact none of his police colleagues, including his district inspector, thought his actions were over the top “indicates a much deeper cultural issue within the WA Police”.

The disturbing case came to light in a Pilbara court when two allegations against the teenager of assaulting a public officer were thrown out by the magistrate.

Ms Ridley ruled the boy had been subjected to “unlawful” acts by the police, and was entitled to defend himself.

During a trial this year the boy, who cannot be named to protect his identity, was accused of striking and spitting at Sen. Const. George, at the South Hedland office of the Department of Communities, and then at the police lock-up.

Crucially, there was CCTV of the incidents.

In her ruling, the magistrate said the footage showed that despite never being formally placed under arrest, the boy was repeatedly pushed, jostled, threatened with a closed fist, grabbed by the arm and the shirt.

“The accused was pushed with such force that his back and head hit the back cushion of the chair,” she said.

“The only acts of aggression and violence to that point were by Sen. Const. George against the accused.”

Ms Ridley also found the CCTV vision was in stark contrast to the sworn evidence provided by Sen. Const. George and another officer, Sen. Const. Michelle Giles.

“Having viewed the CCTV a number of times, it is incomprehensible how any right-thinking member of the community could ignore, fail to see or simply forget the violence being perpetrated against a child either by them or directly in front of them,” Ms Ridley said.

“Barely any of their recall evidence is accurate, beyond where they went and who they were with.

“They were not credible witnesses.”

It was also revealed that on instruction from Pilbara District Insp. Neville Dockery, Sen. Const. George viewed the CCTV before giving evidence— then changed his account substantially.

“The showing of the CCTV ... was unacceptable and had the potential to result in these charges being permanently stayed,” Ms Ridley said.

In throwing out the charges against the boy, Ms Ridley said it was deeply troubling that police apparently believed such aggression was reasonable.

She urged them to have the “maturity to address, rather than deny the underlying issues which exist within its ranks”.

“These are not ‘isolated incidents’ as they would have the community believe,” she said.

In a statement yesterday, WA Police said the case was “complex and sensitive in nature”.

“The comments made by the magistrate are being taken very seriously,” WA Police said. “They will form part of an internal investigation that has already commenced into how police interacted with the 13-year-old boy.”

Both officers are still at work, with one in Port Hedland.

In the written judgment, Ms Ridley was scathing of the Department of Communities staff who called police, saying they had “abandoned” the boy, and “failed him spectacularly”.

“The department criminalised his behaviour rather than use it as a tool to teach him emotional control,” she said.