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Less than three months ago, ailing rugby great Jonah Lomu told a British newspaper it was his aim to see his two young sons turn 21.

On a dialysis machine for the last four years after his kidney failed there were no promise he would be there when his sons Brayley, six, and Dhyreille, five, officially come of age.

"My goal is to make it to the boys 21sts," he told the Daily Mail. "There are no guarantees that will happen, but it's my focus."

On Wednesday, aged 40, he collapsed and died.

It came as a shock not only to rugby fans but also those who had seen him recently at the Rugby World Cup.

Former Wallabies half-back George Gregan said Lomu had been in the best shape he had seen him for a long time at the tournament.

"He looked the best I've seen him in many years because I've seen him for a number of years at Sevens tournaments and around other rugby events and he just had that sparkle and that look of life in his face ... I'm totally shocked by this news," he said.

Prime Minister John Key who had attended a charity dinner in Lomu's honour in London during the Rugby World Cup last month said he appeared in good spirits.

"He said to me actually from a health perspective he was feeling better than he had for a long period of time," he said.

Lomu quit rugby in 2003 aged 27 because of Nephrotic syndrome, a rare and serious kidney disease and had a transplant in 2004.

But in 2011 his body rejected the transplant and he was underdoing dialysis six hours at a time, three times a week since then.

"You have to try and stay up and be happy and positive about it," he told the newspaper.

"It does get you down at times. It's difficult. Every dialysis patient is different but we have one commonality: we have no other choice. Your second choice isn't really a choice. It's just you giving up.

"This disease is a challenge but you either lie down and die or you accept it and carry on."