It was well-known there was a high risk of exertional heat stroke and heat illness in the first five days after players restarted training, specialist acute-care physician Simon Quilty said.

"That extends out to 14 days, so it makes sense that a 14-day period of acclimatisation should be a standard across all sporting codes, not just professional sporting codes," he said.

Dr Quilty was giving evidence on Wednesday at the NSW inquest into the death of Manly Sea Eagles player Keith Titmuss.

The 20-year-old suffered a seizure after a cardio workout at the club's base at Narrabeen, on Sydney's northern beaches, on November 23, 2020.

He died in hospital five hours later.

Titmuss most likely suffered exertional heat stroke, although an autopsy was unable to confirm the exact cause, the inquest heard previously.

An arterial blockage could also have compromised the young player's heart health, making it more difficult for him to recover from heat-related illness, Dr Quilty said.

The expert spent several years working in the Northern Territory and published a study in 2021 suggesting only three per cent of heat-related deaths were identified as such.

"I don't feel clinicians and first responders are adequately trained in recognising the implications of heat on illness," Dr Quilty said.

But recent statements from the NRL's chief medical officer Sharron Flahive showed some progress on the issue, he added.

"There's a greater appreciation of the extent heat contributes."

Family and supporters watched on in the Sydney courtroom wearing black shirts bearing the number 623 on the back, Titmuss's posthumously awarded Manly Sea Eagles first-grade player number.

Dr Quilty offered them his condolences.

"It's a substantial loss for such a young bloke," he said.

The inquest continues on Wednesday, when a statement is expected from Titmuss's family.

Australian Associated Press