An unarmed and vulnerable Aboriginal woman died after three police officers in Perth restrained her for longer than necessary and failed to monitor her breathing, a coroner has found.
Cherdeena Wynne, 26, died in April 2019 at Royal Perth Hospital, five days after she was pinned face-down on the side of a busy highway for almost two minutes.
She had walked into traffic while in the grips of a severe mental health episode before being restrained, having had several earlier interactions with police.
West Australian coroner Phillip Urquhart found Ms Wynne, who was not wanted over any criminal behaviour, had gone into cardiac arrest when restrained.
He found she had methamphetamine in her system and may have experienced positional asphyxia, the risks of which are greatly increased by placing a restrained person in a prone position.
In inquest findings published on Wednesday, Mr Urquhart said one officer - Sergeant Jace Williams - had pinned Ms Wynne to the ground with his leg across her upper back while two others assisted.
CCTV footage showed the 115kg officer maintained that position for one minute and 42 seconds, despite it only taking 45 seconds for his colleague to place Ms Wynne in handcuffs.
By the time she was raised to her feet, the mother of three was limp, her head was slumped forwards and her eyes glazed over.
She never regained consciousness.
Sgt Williams told the inquest he continued to pin Ms Wynne after she was handcuffed because he mistakenly believed she had a syringe in her hand.
It was in fact a cannula she had removed from her arm during an earlier interaction with police and paramedics.
The continued restraint was unnecessary and inappropriate, the coroner found, particularly given the officer knew the dangers of positional asphyxia.
"I am at a loss to understand why that was done," he said.
But Mr Urquhart said it was impossible to say whether the "unjustified" delay contributed to her cardiac arrest because it could not be definitively established when she had stopped breathing.
He said it was deeply troubling that none of the officers had adequately monitored Ms Wynne's breathing despite being trained to do so.
The coroner also criticised the officers' testimony regarding the duration of the restraint which he said was contradicted by the CCTV footage.
Police officers should be on notice that such discrepancies could have a significant bearing on assessments of their credibility, he warned.
Ms Wynne had been handcuffed by police earlier in the day during a domestic dispute at her mother's unit.
At least one attending officer had seen a note indicating she required a welfare check, having absconded from a mental health unit the previous week.
But there was no evidence any of the attending officers inquired about her mental health.
The coroner found Ms Wynne, a Noongar Yamatji woman, had been a victim of domestic violence and battled ongoing mental health issues.
Her father had also died in police custody at the same age 20 years earlier.
Her grandmother Aunty Jennifer Clayton said the family was heartbroken to learn of the failures in the lead-up to Ms Wynne's death.
"She committed no crime, she was confused and just needed help," she said.
National Justice Project solicitor Karina Hawtrey said she was disappointed the coroner's only recommendation related to the need for further police training.
"The recurring problems of a lack of safety for First Nations people and those with mental health issues in their interactions with police should have been addressed," she said.
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Australian Associated Press