A man who was losing consciousness while in a restraint chair in police custody died the next day.
Now the Independent Police Conduct Authority has found police in the Counties Manukau Custody Unit failed in their duty of care to Jaye Taueli.
Taueli was arrested in August 2021 for breaching bail conditions.
Officers were told he had consumed methamphetamine prior to his arrest and when he arrived he was assessed and placed in a cell.
A short time later, Taueli began banging his head on the cell door and wall, and then attempted to strangle himself.
He was placed in a restraint chair to prevent him from self-harming.
The police watchdog said the use of the chair was justified, but less forceful and less intrusive options should have been used first.
Once in the restraint chair, Taueli forcefully thrashed his head back and forth over 160 times.
A doctor was called as the custody sergeant believed Taueli might need to be sedated to calm him down.
He then began to lose consciousness, and officers believed him to be asleep, failing to recognise he was becoming unresponsive.
Once the doctor arrived an ambulance was ordered, however, he did not regain consciousness and died the following afternoon.
His death was determined to be caused by a pre-existing vascular lesion rupturing, leading to a brain bleed.
Authority chair Judge Kenneth Johnston KC said the custody sergeant failed in his duty of care.
"Detainees are reliant on officers to provide them with care while they are in custody. They are often in a high-risk, vulnerable position, and police staff are the only people available to assist them.
"In Mr Taueli's case, none of the officers recognised that he was suffering a medical event and becoming unresponsive. Police need to ensure custody and frontline staff are adequately trained to recognise levels of consciousness."
Police have acknowledged the findings, and said they have since made several changes to processes.
Relieving Counties Manukau district commander Inspector Matt Srhoj said police took thousands of people into custody every year and they were more often than not held without incident.
"This was a tragic incident, and my deepest sympathies remain with Mr Taueli's family," he said.
"The management of people in our care is something we take very seriously; they are often vulnerable and require a high level of monitoring and support.
"Our focus is on caring for people as individuals and ensuring there are processes and systems in place to support this approach."