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 man, whose name and age wasn't given in the report, died of suffocation related to a methamptamine overdose on November 13.

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.

Sometime 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.

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 Chair Judge Colin Doherty said police policy existed precisely 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.