The inquest to the 36-year-old's fatal asthma attack began on Monday with questions remaining about the quality of care the father-of-one received while in a minimum-security wing of a Sydney jail.

Fellow inmate Jeremy Preo broke down in tears as he described trying to help Mr Reynolds, who went stiff while struggling to breathe on the night of August 31, 2018.

Trained in first aid, Mr Preo placed Mr Reynolds in the recovery position and did his best to open the man's mouth and airways.

"He's done like a big gurgle and that's when I started rubbing my arm on his back and said, 'mate keep breathing'," Mr Preo told the NSW Coroners Court, wiping his eyes.

"He was alive then. He could hear me."

Minutes later, about 11.49pm, the only registered nurse at the prison that night, Kasey Wright, arrived on scene.

Mr Preo said he was relieved and told her Mr Reynolds was breathing but had some yellow fluid coming out of his airways.

But he was distressed after seeing her roll Mr Reynolds onto his back.

"I've seen her ... slap Nathan in the face and aggressively shake him, and telling him to wake up," Mr Preo said.

"The nurse then said to (prison officer John Fifita) that Nathan's had a drug overdose."

After Mr Fifita asked what drugs Mr Reynolds had taken, Mr Preo retorted: "How can you say that after you've sat there ... and watched him have an asthma attack?"

Ms Wright is due to testify about her version of events later in the inquest.

CPR began shortly after Ms Wright's arrival, continuing when paramedics showed up about 12.14am.

Thirty minutes later, and 77 minutes after he first radioed prison guards about his breathing difficulties, Mr Reynolds was declared dead.

He had earlier been seen seated on a couch next to Mr Fifita taking short breaths and puffs of Ventolin.

"He wasn't (talking). He was in a rhythm with his breathing," Mr Preo said.

When he said Mr Reynolds needed to be taken to the medical centre and put on oxygen, Mr Preo said prison officers replied they couldn't administer medical treatment.

The inquest is expected to hear from respiratory specialist Greg King that when Mr Reynolds radioed for help at 11.27pm, his condition was already severe and life-threatening and the window to prevent his death was very narrow.

Professor King is expected to say Mr Reynolds was likely in respiratory arrest and needed to be administered oxygen by mask and either nebulised Ventolin or a puffer with a spacer, counsel assisting Chris McGorey said in his opening address on Monday.

Ms Wright wasn't called until after 11.40pm, when the prison officers arrived.

Mr McGorey said records at the South Windsor prison showed Mr Reynolds had been issued with Ventolin inhalers at least six times.

From time to time, he may have also borrowed and used other inmates' puffers and been issued puffers by staff who failed to document the event, Mr McGorey said.

Noting about 400 Australians die each year of asthma, the barrister said preventer medicines have reduced attacks but the challenge remained in identifying those at risk of severe attacks.

Inmates, nurses, guards and a doctor who treated Mr Reynolds are expected to give evidence.

Mr Reynolds' sisters, who attended Monday's hearing alongside her brother's partner and mother, said they want truth and justice.

"Nathan died on the cold floor of a prison, with no loved ones around him. He was just 36 years old - he died far too young," Taleah and Makayla Reynolds said in a statement.

Australian Associated Press