An Indigenous inmate at Sydney's Long Bay prison died after being restrained by riot officers and tranquilised, according to documents obtained by the ABC.

Key points: Kempsey man David Dungay, 26, died in Long Bay jail in December 2015

Kempsey man David Dungay, 26, died in Long Bay jail in December 2015 A report into Aboriginal deaths in custody shows he died after being sedated and restrained

A report into Aboriginal deaths in custody shows he died after being sedated and restrained NSW Corrective Services says his death was not suspicious

NSW Corrective Services says his death was not suspicious Mr Dungay's family says he was "murdered"

A copy of a New South Wales Health Department report into an Aboriginal death in custody shows inmate David Dungay, 26, died after being restrained and tranquilised, despite him saying he could not breathe.

Corrective Services maintains police are not treating the death the Kempsey man as suspicious.

But in their first interview since he died in the jail's mental health unit on December 29, 2015, Mr Dungay's family has described his death as "murder".

They are concerned at revelations that staff were instructed to clean blood from the floor of his cell before investigators arrived.

His mother Leetona Dungay wants those present at the time of his death to be investigated.

"[It's] straight out murder. They murdered my son. They've got to be accountable for it," she said.

"They've got to come to justice, whoever has done this to my son."

Staff told prisoner 'if you can talk, you can breathe'

The sequence of events leading to Mr Dungay's death began when the diabetic was ordered by Corrective Services staff to stop eating biscuits in his cell.

According to the Health Department's report, he became aggressive and Corrective Services staff called in the Corrective Services Immediate Action Team, which deals with riots in the jail.

The report said Mr Dungay had a history of violence towards staff and the decision was made to move him to a cell with CCTV cameras.

The report says Mr Dungay was given a powerful sedative. ( Supplied )

It said that Mr Dungay resisted being moved and started yelling that he could not breathe.

He was told by Corrective Services staff that if he could talk he could breathe.

After being escorted to the second cell, he was held face down on the mattress and nursing staff were called to sedate him.

According to an autopsy report also seen by the ABC, shortly after an injection of the powerful sedative midazolam, his face turned purple and he stopped breathing.

Staff and ambulance officers attempted to resuscitate Mr Dungay for 40 minutes without success.

The report reveals that nursing staff were instructed to clean what is described as "a small amount of blood" from the floor of Mr Dungay's cell.

This was done before police investigating the death arrived.

Calls for prisons to improve inmates' health services

Mr Dungay was serving an eight-year sentence for home invasion and sexual assault.

He was being treated in the jail's mental health unit after being diagnosed with chronic schizophrenia with acute psychosis.

He was due to be released on parole within weeks.

His family's lawyer, George Newhouse, said Mr Dungay's case raises a number of issues that need to be addressed by the coroner.

"Aboriginal people have an extremely high rate of diabetes, but the management of diabetes in the prison system seems to be problematic," he said.

"David's case is possibly an example of that.

"The treatment of Aboriginal people in prisons and their health needs is a critical issue in investigating his death."

According to the NSW state coroner, Mr Dungay was one of 26 deaths in custody last year and one of six indigenous deaths in custody.

Health Department raises issues

The Health Department's report into the incident raised a number of issues, including:

Sorry, this video has expired Mr Dungay's family says he was murdered.

• Health staff did not assess Mr Dungay's airway, breathing and circulation or vital signs before administering the sedative as per the Enforced Medication and Rapid Tranquilisation policy

• Emergency resuscitation equipment was not available immediately before the medication was administered

• The antidote to the sedative was not available as per policy

• Only two of the four health staff responding to the incident documented their role in the Health Record

• The report authors believe the subordination of health staff to Corrective Services staff in the hospital needs to be explored

The Health Department's Justice Health and Forensic Mental Health Network said the recommendations to improve patient care had been "actioned".

An 'unnatural death'

An initial autopsy did not determined a direct cause of death.

The former clinical director of the NSW Department of Forensic Medicine, Sydney University Professor Johan Duflou, said it was clear Mr Dungay did not die of natural causes.

"An unnatural death is any death where there has been involvement by some external agency or force," he said.

"In this case, conditions that we'd have to consider is restraint of the patient."

In a statement, Corrective Services Commissioner Peter Severin said he deeply regretted Mr Dungay's death.

He said that preliminary indications from police suggested there was no criminal negligence.

An inquest date is yet to be announced.