A Manly Sea Eagles player who collapsed and died following a training session with the NRL club in 2020 was killed by exertional heat stroke, an inquiry has heard.

Keith Titmuss had a body temperature of 41.9 degrees Celsius when paramedics arrived at the scene at the Sea Eagles' base in Narrabeen on November 23, 2020.

Mr Titmuss had just completed a two-hour training session with the club when he started to become agitated and then suffered several seizures.

The 20-year-old was rushed to Royal North Shore Hospital where he suffered a fatal cardiac arrest.

A coronial inquest into Mr Titmuss's death began on Monday.

An autopsy report was unable to determine the cause of death, but counsel assisting the coroner Adam Casselden said medical experts had since concluded that the young player died due to exertional heat stroke.

Mr Casseldon told the court that after emergency services arrived at the scene, paramedics took Mr Titmuss's body temperature three times because they couldn't believe it had reached almost 42C.

"I did not believe it. I'd never seen a temp like this before," one paramedic said in their submission.

The maximum air temperature was 24.9C the day Mr Titmuss collapsed after training.

Mr Casselden told the court that the 20-year-old had gained weight since the previous NRL season and was not as fit as many of his teammates.

"The fact is that Keith, relatively speaking, was less fit and less conditioned to the heat than he would have been other times," Mr Casselden said.

Training occurred in 'hot' building but Hasler says it wasn't 'strenuous'

Some of the training that day also occurred inside in a building on site at Narrabeen, which was not air conditioned.

A paramedic described the building as "so hot" after he arrived at the scene to treat Mr Titmuss.

Distinguished professor of sport and exercise science at the University of Technology Sydney, Aaron Coutts, is one of several experts who submitted evidence to the coronial inquiry.

In his submission, Professor Coutts said the training on November 23 was "notably demanding" and the level of intensity was "inappropriate" given it was first training session of the pre-season period.

A submission from then Sea Eagles coach Des Hasler contradicted this.

Mr Hasler said the session had "not been overly strenuous or taxing".

Titmuss remembered for his 'gentle and kind nature'

Mr Titmuss was a rising star in the Manly Sea Eagles before his death, scoring the winning try in the 2017 Holden Cup.

The 20-year-old had been admitted as a member of the club's top-30 NRL squad just days before he collapsed.

He was remember by his family as a "kind" and "easy going" young man who loved to joke around.

"He was much loved for his gentle and kind nature," Mr Casselden said.

"His natural gifts and his beautiful personality are deeply missed."

The inquiry continues.