A man lay dead in a Hawke's Bay police station cell for nearly six hours with a fresh tray of breakfast beside him before police realised he had died.
The Independent Police Conduct Authority (IPCA) on Thursday released a report into the circumstances of the man's death in the Hawke's Bay Custody Unit in November 2017.
It found that police as an organisation, as well as individual officers, failed in their legal duty of care of the man.
The 39-year-old man died of suffocation related to a methamphetamine overdose on November 13.
Eastern District Commander Superintendent Tania Kura said she was"very disappointed that on this occasion we did not follow the standards and policies established to keep detainees safe".
"This was a tragic incident and my deepest sympathies remain with the man's whanau."
She said the staff involved had acknowledged they failed to adhere to the procedures.
IPCA said the findings of its investigation raised "serious concerns with the evaluation and monitoring of the man during his detention, and with the training and supervision of custody staff".
Police said in a statement following the report's release that a number of substantial changes had been introduced to strengthen practice and procedure when dealing with detainees at the Hawke's Bay Custody Unit (HBCU).
The man was taken into police custody at the Hawkes Bay Area Custody Unit early on November 12, 2017.
When the man was received into custody, he resisted police attempts to search him, and his health and wellbeing were not properly evaluated due to his agitated state.
This led to police failing to become aware of warnings on the man's file, particularly that he had suffered a brain injury in the past and did not take his prescribed medication to prevent seizures.
Some time during the night of November 12, 2017, the man took a large dose of methamphetamine.
In the early hours of November 13, he suffered prolonged and increasingly violent seizures.
A post mortem later revealed the man had a fatal level of methamphetamine in his system, and expert medical advice was that he had died of suffocation related to a seizure, probably about 4.30am.
DISCOVERED WHEN OFFICER TRIED TO WAKE MAN FOR COURT
Throughout the period of his detention, police failed to make regular checks on the man as required by policy, the IPCA report found.
Several checks were recorded as having been made while the man was having seizures, and after the man had died.
An officer placed a breakfast tray in the man's cell at 5.42am on November 13, yet the man's death was not discovered until about 10am that morning when an officer tried to wake the man to take him to court.
The IPCA found that officers in the custody unit repeatedly failed to perform their duty to care for the man as required by law and policy. Although these omissions were not causative of the man's death, they were serious and inexcusable.
Authority chairman Judge Colin Doherty said police policy existed precisely to avert the sort of outcome that occurred in this case.
"The omissions of officers to comply with that policy were likely to cause injury or suffering to a vulnerable adult such as this man.
"Poor leadership, supervision, and support of custody staff contributed to a culture in the custody unit that tolerated a repeated and serious disregard of police policy and good practice."
The IPCA found that there was insufficient evidence to support a criminal prosecution of any individual officer, and that police could not be held criminally liable for the potential Crimes Act offences identified.
It noted an organisation such as police could be held criminally liable for the actions of staff who fail to fulfil their obligations under the Health and Safety at Work Act 2015.
The IPCA could not determine how the man had accessed the methamphetamine while in custody.
Superintendent Kura said police had taken the report findings "extremely seriously, in particular, the concerns raised around the operation of the Custody Unit and the care provided to detainees.
She said the IPCA had recognised the extensive changes that have been implemented in the custody unit since the death.
There had been improvements in training and induction procedures for staff working in the custody team, and for how detainees are assessed, monitored and cared for, Kura said.
New scanning equipment has also been introduced to assist staff searching detainees and they now have the expertise of a mental health nurse working alongside their custody team from Monday to Friday, Kura said.
"The custody unit can be a challenging environment to work in as staff are asked to support people who are in a very vulnerable and often highly charged state.
"We are a values-based organisation and one of those values we work to is empathy.
"This needs to be displayed in our custody units as we work to care for and support those people who are detained with us."
A Custody Review Working Group had also been created to monitor the quality of custodial care environments across the Eastern District, as well as supporting the staff who work in challenging conditions, she said.
Police said the matter was referred to WorkSafe, but they had been informed that it did not intend to investigate.