"It's a big red flag there's something really going wrong with his asthma," respiratory specialist Greg King told the 36-year-old man's inquest on Wednesday.

"It's not controlled."

The regular issuing of puffers probably should have been enough on its own to prompt medical staff at the Outer Metropolitan Multi-Purpose Correctional Centre to recognise Mr Reynolds' asthma control was "non-existent" and he needed an asthma plan, Professor King said.

The plasterer, 36, died shortly after midnight on September 1, 2018, on the floor of his wing in the minimum-security prison.

His inquest has heard no asthma management plan was in place for the father-of-one, despite prior hospital admissions and the heavy use of puffers.

Prof King, who researches the lung condition affecting 2.7 million Australians and treats patients in Sydney, said puffers could last some asthmatics six months.

Those under very good control will very rarely use Ventolin, even less than once a month, he said.

Preventer medication was a critical step in asthma management, as was explaining to a patient how it worked and why it was important.

Prof King said medical staff would have ideally done supervised or regular checks of Mr Reynolds' preventer use in the months before the fatal attack.

Mr Reynolds reported breathing difficulties and his need for a nurse at 11.27pm on August 31. Prison officers arrived at his wing at 11.40pm, verified the report and called for a nurse.

Even if prison officers had given him oxygen at 11.40pm, it likely wouldn't have altered the outcome, Prof King said.

"The underlying problem is his lungs are progressively shut off during the evolution of this attack," he said.

"Giving oxygen will only temporise things for a short number of minutes at most."

He said advanced life support care, like that from paramedics or in a hospital, was necessary as ventilating the lungs was the critical issue.

Mr Reynolds' condition would have likely begun deteriorating from when he first experienced cold symptoms and very likely worsened throughout the day of August 31, the expert said.

Coroner Elizabeth Ryan also heard on Wednesday from those who were closest to the tradesman.

His daughter told of how her father was the reason she loved the outdoors and playing sport.

"My dad was creative and fun and the biggest Aussie ever," she said.

His stepdaughter teared up about how her "bestie" and the one who was "always just there" wouldn't see her graduate or get married.

Former partner Karen Pochodyla read a letter he sent the family during a previous stint in prison, in which he tells the family how he's eating well, transforming himself.

He also encourages them to be good to their mother.

"Please make things easier on her ... I'm to blame for the sad bit, mostly, so be the happy bit," he said.

Taleah Reynolds said her older brother understood the value of "simply spending time with people" and made a profound connection with each family member, from grandparents to stepkids.

"In return, they all loved Nathan deeply," she said.

He loudly and proudly celebrated his kids' academic and sporting achievements and had described his biological daughter as his "best achievement".

"Losing Nath has been incredibly hard ... but loving him was easy," Ms Reynolds said.

"He's more than a statistic, he's more than just a hashtag."

The inquest was adjourned until December 11, when another medical expert will give evidence. Findings are expected in 2021.

Australian Associated Press