Jonah Lomu died from a blood clot on his lung, possibly as a result of his long haul flight home from the UK.
Former All Blacks doctor John Mayhew told the BBC that Lomu would have felt little pain when he died suddenly last week.
Mayhew spoke to BBC Five Live last night and said: "He returned from the UK via Dubai and appeared to be in good health before he died."
"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."
Mayhew said the other possibility was cardiac arrest, but "on the balance of probabilities it was probably a massive clot on the lung."