The police watchdog, the Independent Police Conduct Authority, says a man high on a cocktail of alcohol and drugs might have lived had he been properly checked while in a police station cell.
The authority says it was clear that Francisco de Larratea Soler, 43, had been dead for some time before he was found.
Police have admitted they gave inadequate care to the man who died in the Whakatane station cell in December 2008.
Mr de Larratea Soler was picked up in an extremely drunk and disorientated state and hours later was found dead in the cell.
Officers did not enter his cell during the seven and a half hours he was in custody. A post-mortem concluded he died of methadone toxicity.
The authority's report, released on Friday, says the case was marked by omissions and failures by police.
Mandatory checking not observed
The authority finds that police risk evaluations were flawed and inadequate, Mr de Larratea Soler went unchecked in his cell for too long, and written procedures were out of date. There was also no complying with policies on mandatory checking of prisoners.
In addition, managers are faulted for inadequate resourcing to allow proper oversight of the watchhouse and cells at Whakatane police station.
The authority says Mr de Larratea Soler's death may have been inevitable, but its occurrence in police custody was avoidable.
In a statement on Friday, police say the officers involved were busy with other duties and did not recognise the risk in the case.
Changes to the policing manual are being made as a result, as well as more staff training and better oversight of the station's watchhouse.
Police say neither disciplinary action nor criminal charges have followed from an internal review of the case.
A friend of Mr de Larratea Soler, Grant Stanley, says officers' handling of the case was massively negligent and he's angry no officer has been disciplined or prosecuted.
Mr Stanley says police should apologise for their actions.