Jai Davis (30) died in a prison cell in February 2011 from a suspected drug overdose after arriving at the Milburn facility with drugs concealed internally.
Police yesterday made a decision not to prosecute following a review into evidence given at last year's coroner's inquest into Mr Davis' death.
The coroner's inquest in Dunedin last year heard of multiple failings by staff, including not calling a doctor despite Mr Davis appearing unwell.
Recorded telephone conversations between Mr Davis and a prisoner had made it clear he planned to bring drugs into the prison.
Since her son's death, Victoria Davis has been fighting for Department of Corrections staff to be held accountable.
Ms Davis said she was ''devastated'' when Inspector Steve McGregor, of Southern District Police, called her yesterday morning and told her a decision had been made not to lay charges.
She believed two key Corrections officers and two nursing staff responsible for his care should have been charged, along with management at the facility.
''It leaves a bad taste in the mouth when there has not been accountability.
''These people are still working there and these nurses are still able to be nurses.''
Her son had been treated as a ''subhuman'' and the decision by police sent a message to Corrections staff they could get away with poor treatment.
The decision was a case of one government department (police) protecting another, she said.
''Put it this way, if you left your child looking like a corpse, sunken-in eyes, vomiting [and with] stomach pains, you would be had up for it.''
The decision was a blow but Ms Davis vowed to continue fighting for justice and would look at taking civil action against Corrections as a whole and private criminal prosecutions against individuals.
''I will be looking at every avenue, because there needs to be accountability.''
A decision on civil action would be taken after coroner David Crerar issued his finding.
Without proper accountability, she feared history would be repeated.
Insp McGregor said in a statement yesterday the Crown had completed a review of evidence presented at the coronial hearing in December last year and also of all previously available evidence.
''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,'' he said.
When contacted by the Otago Daily Times, he ''totally'' rejected Ms Davis' suggestion police were protecting another government department.
''It wouldn't have mattered who it was; if someone was criminally liable, we would be charging them. It's irrelevant it's another government department.''
He was not prepared to go into detail over why police chose not to prosecute, saying it was as a result of a legal opinion they had received from the Crown.
The fact three and a-half years had passed since the death did not play a significant role in the decision.
Corrections chief executive Ray Smith issued a statement yesterday saying it acknowledged the decision not to lay charges.
''Corrections has implemented all recommendations arising from its own reviews and has met with Mr Davis' mother to formally apologise for aspects of his care that fell below the standard expected.''
It was awaiting the coroner's finding and would not answer further questions until it was released.
