The two-week inquest into the death of Aboriginal man and Type 1 diabetic David Dungay, 26, at Long Bay Hospital in December 2015, began before Deputy State Coroner Derek Lee at Sydney's Downing Centre on Monday.

Counsel assisting the coroner, Jason Downing, said Mr Dungay was in the prison hospital ward as he "had been assessed as a mentally ill person" and also required daily injections of insulin.

Mr Dungay twice spent time in the exercise yard on the day of his death, during which he ate "what appeared to be some crackers" and grabbed another packet of biscuits, believed to be rice crackers, on his way back to his cell in the afternoon.

At about midday, and again at 2pm, he was recorded as having a high blood sugar level but was considered "asymptomatic" by a nurse, the court heard.

It remains unclear who asked Mr Dungay to not take the biscuits into his cell because the additional food may have an adverse effect on him, Mr Downing said.

But when he was asked to hand them over, it appears he "began to stuff them" into his mouth, he said.

Two prison officers responded and asked the six-member Immediate Action Team to help move Mr Dungay to a different cell with a camera in it.

The court was played footage showing Mr Dungay being transferred to the cell where he was administered an intramuscular injection, or sedative.

He is restrained by at least five IAT officers in the prone position, or facedown on a cell bed, spits blood and repeatedly screams "I can't breathe" to which one officer replies: "You're talking, you can breathe".

Mr Dungay became unresponsive, went "limp" and vomited, and could not be resuscitated, Mr Downing said.

"I anticipate that the evidence will indicate that five of the six IAT members had not undertaken any training in respect of positional asphyxia risk and had no effective knowledge of it," he said.

The inquest continues.

Australian Associated Press