Kato Ottio's sudden death is thought to have been caused by severe heat stroke, the hospital where he was treated in Papua New Guinea has revealed.
Ex-Raider Ottio, who was due to travel to England on Thursday to begin his contract with Super League club Widnes, died at the Pacific International Hospital (PIH) in Port Moresby in the early hours of Tuesday morning.
He had earlier collapsed after taking part in an eight-kilometre road run with his former PNG Hunters teammates.
PIH is reportedly recommending a postmortem to confirm the cause of death.
"Kato was brought to PIH in a critically ill condition which appeared to be complications of a possible severe heat stroke," the hospital's acting medical director Dr Paul Alexander told local newspaper Post Courier.
"He was promptly attended to at the PIH emergency room by the ER duty doctors and the critical care specialist.
"Extensive diagnostic work-up including a chest X-ray, CT brain and several blood tests were done to continuously monitor the patient's condition."
Ottio's manager Steve Deacon told Wide World of Sports that the 23-year-old had decided to continue running after the rest of the squad had finished.
"The run was over and a few of the boys walked back while others got a ride," said Deacon.
"But Kato told a few of the players 'let's run back' - he wanted to use the professionalism he gained from his time at Canberra to rub off on his younger countrymen.
"He got slower and eventually collapsed. He was unconscious and in a coma and they were not able to revive him.
"He was a great guy and I'm just devastated."
Ottio was an ever-present at the recent World Cup as Papua New Guinea won their group before losing to England in the quarter-finals.
He was set to travel to Widnes and share a house with international teammate Wellington Albert, who was expected to delay his departure as he comes to terms with the tragic news.