Rugby great Jonah Lomu probably died from a blood clot that formed during a long-haul flight to New Zealand, one of his medics said Monday, as plans were unveiled to honour the legendary winger with a public memorial.

Former All Blacks doctor John Mayhew, who helped treat Lomu's chronic kidney disease, said a flight-related clot was the most likely explanation for his shock death in Auckland last week aged just 40.

Mayhew, a close family friend who announced the star's death to the world, said Lomu's kidney disease made him vulnerable to such a scenario.

The player had just returned to Auckland after seeing his beloved All Blacks win the Rugby World Cup in Britain, a marathon flight, even with a stopover in Dubai.

"He returned from the UK via Dubai and appeared to be in good health before he died," Mayhew told the BBC.

"We think the most likely cause was a clot on the lung which can be a complication of long distance travel. Jonah was at greater risk of that happening because of his renal condition."

He said Lomu, an electrifying talent who became the game's first global superstar, would not have known what hit him.

"I think it was instantaneous. He was unaware of what had happened," he said.

"It's just one of those tragic complications that can occur in people with chronic renal conditions."

The US Center for Disease Control advises on its website that long-distance air travel can increase the risk of venous thromboembolism (a blood clot in the vein) by two-to-four times, more if there are pre-existing conditions.

Many airlines advise passengers to walk around the flight cabin or wear compression stockings to help prevent clots forming.

Lomu's passing has prompted tributes from not only from the rugby world but also Hollywood celebrities, charities and Britain's Queen Elizabeth II.

Such has been the outpouring of grief in New Zealand that authorities said Monday they had decided to hold a public memorial service at the 50,000-capacity Eden Park stadium in Auckland.

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Prime Minister John Key said large numbers of people wanted to pay their respects at the November 30 service.

"Jonah touched people's lives across the country and around the world," he said.

"This service will be open to all members of the public who want to remember the significant contribution Jonah made, not only to rugby here and overseas, but also to the wider community through his work with charities."

The memorial service will be followed by a private funeral the next day, December 1.