No charges will be laid in relation to the death of a man in Otago Corrections Facility four years ago.
Jai Davis, 30, died in a prison cell in February 2011 from a suspected drugs overdose.
A review into evidence given at an inquest last year found there was "no new evidence that would give rise to any criminal liability which could lead to a prosecution", Inspector Steve McGregor of Southern District Police said today.
"The Crown has completed a review of the evidence presented at the coronial hearing in December last year and also a review of all previously available evidence," Mr McGregor said.
"The review found that the evidence fails to meet the standards required by the Solicitor-General's prosecution guidelines and that no charges against any individual should be laid."
Mr Davis' family have fought for a prosecution in relation to his death, with his mother Victoria Davis telling the coroner's inquest in December that Department of Corrections staff had failed her son in their duty of care.
She told the court she was concerned that some prison staff had been "so indifferent to his wellbeing" that he had "unnecessarily died".
Mr Davis had drugs inside him when he went into the Otago Corrections Facility as a remand prisoner in February 2011.
Although Corrections staff knew that from an intercepted phone call, he was placed alone in an at-risk cell and did not receive medical help. He died in the cell two days later.
Roger Brooking, alcohol and drug counsellor and prison reformer, said the decision not to prosecute was disappointing but typical of an inherent systemic bias.
Mr Brooking said Detective Senior Sergeant Colin Blackie, initially in charge of the investigation, "made it very clear" he would have launched prosecutions.
But Mr Brooking said Mr Blackie was taken off the case and his replacement, Detective Inspector Steve McGregor, was "higher up the food chain" and that changed the investigation dynamics.
Mr Brooking said he would support Mrs Davis if she decided to launch a private prosecution.
"In the last ten years approximately 90 prisoners have died unnatural deaths and the police have never prosecuted anybody," Mr Brooking said. "This is just part of a pattern.
"I can't see anything changing unless or until somebody is actually prosecuted."
Mr Brooking said Corrections repeatedly ignored coronial recommendations, or responded with hollow promises.