"The sky was red everywhere. I didn't see a mushroom cloud, just crimson wherever you looked, it was so vivid," he tells AFP inside his Sydney home, spreading his arms to convey the enormity of the vision.

"I won't forget until the day I die the colour of the sky that day."

At the time the Nagasaki nuclear bomb went off on August 9, 1945, Uren was a 24-year-old prisoner of war (POW) who had experienced the brutality of the Thai-Burma railway before being sent to Japan as a slave labourer.

Uren, now 84, was working in a copper factory smelting down looted Chinese bronzes for the Japanese war effort at Imuta, about 80 kilometres (50 miles), from Nagasaki, when the A-bomb went off.

He says the impact of the blast on his captors was almost immediate.

"The Japanese guards came out and said 'big boom' then started bringing out Red Cross parcels they'd held back," says Uren. "The war was over."

The nuclear explosion and his earlier experiences in the jungle death camps working on the notorious Hellfire Pass were defining experiences for Uren, who before the war had been a heavyweight boxing contender and narrowly lost a bout for the Australian national title.

He became a committed pacifist, who after the war campaigned for social justice and embarked on a political career that saw him appointed a top-level minister in the reformist Gough Whitlam's government of the 1970s and Bob Hawke's Labor administration a decade later.

Born to a working class family in Sydney, Uren excelled at sport from an early age, particularly swimming and rugby union. But it was boxing that became his main sporting love, with the fight game offering a heavyset, six foot two (1.9 metres) youth of limited education and modest means the opportunity to make money at the tail-end of the Great Depression.

As with so many of his generation, the war intervened and Uren signed up in September 1939 as a member of the Royal Australian Artillery.

"I wasn't full of patriotism but I wanted to be able to say at the end of the war that I was a returned soldier," he says.

-- 'We had no idea of the hell ahead of us' --

It was in 1940 as a 19-year-old soldier in Sydney that Uren fought for the Australian heavyweight championship against a veteran pug named Billy Britt, losing on a technical knockout in the seventh round.

He was posted to Darwin in northern Australia and in December 1941 landed at Kupang, on the island of Timor.

The Japanese landed on Timor in February 1942 and after a series of savage battles over three days Uren's heavily outnumbered battalion, originally numbering 1,000 soldiers, was captured after sustaining a casualty rate of 40 percent, including 150 dead.

"We were apprehensive but in a way we were relieved that the fighting was over," he recalls.

"We had no idea of the hell ahead of us."

Uren was initially held for nine months in a camp in Kupang, where he said the Japanese guards, former front-line soldiers, treated the Australians reasonably out of respect for the fight they had put up.

From there he was shipped to the Indonesian capital Batavia, now Jakarta, where he worked on the docks for three months before being sent to the Southeast Asian mainland, where he would experience privations that would change his life forever.

After being held at the Salarang barracks in Singapore for a fortnight, Uren and his comrades were shipped in goods trucks to the Tarsau camp in Thailand.

He said conditions during the week-long journey were inhumane, with temperatures in the unventilated plate-steel goods carts reaching inferno level during the day and plunging to freezing during the night. Most of the 18 months Uren spent working on the Thai-Burma railway were spent at Hintok mountain camp.

Uren, because of his physical size, was assigned as part of a two-man "hammer and tap" crew. It meant the hammer man, Uren, had to bang a sledgehammer into a one-inch (2.5 centimetre) diameter metal rod, held by the tap man, until it drove a hole a metre (3.3 feet) deep in the sandstone, which was then filled with dynamite and blown up to create a railway cutting.

Uren says the crews were initially expected to drive home a single metre-long rod into the rock every day, but as the war wore on they were forced to sink three rods, working 18 hours a day with no breaks.

While the work was back-breaking, Uren says it was his physically smaller comrades who had to transport the dynamited rubble in baskets to create rail sidings who had the hardest job, scrabbling with fully-laden baskets over razor-sharp rock, with feet often protected only by thin cloth swaddling.

Uren says he was beaten on a number of occasions because he stood up to the guards on behalf of colleagues who simply could not go on any more.

-- 'I don't hate the Japanese, I hate militarism and fascism' --

Australian authorities say dozens of POWs were beaten to death during the construction of Hellfire Pass.

"We were subjected to physical and psychological stress which was barely creditable -- malnutrition, multiple infection, inadequate shelter, prolonged marches, work to the point of total exhaustion, capricious violence and humiliation," says Uren.

Only about a third of the 22,000 Australian POWs captured by the Japanese returned home after the war, but Uren says it was the unnecessary losses he saw among other nationalities that helped shape his political philosophy.

He said 400 British POWS arrived at Hintok a few months after the Australians. Instead of giving the most needy the best accommodation, the British allocated officers the most comfortable camp sites, followed by non-commissioned officers, with the lower ranks in the worst spots.

Uren says only 25 of the British group survived cholera and dysentery epidemics during the wet season, while the Australians' survival rate was much higher because under the command of camp leader and physician Edward "Weary" Dunlop they went out of their way to look after their weakest comrades.

"Only a creek separated our two camps, but on one side the law of the jungle prevailed and on the other the principles of socialism," he says.

The prisoners suffered a range of ailments, including malaria, beri beri, pellagra, festering tropical ulcers and, worst of all, cholera.

"You would go out to your workplace and come back that night to find that people with this disease had aged 50 years in a day," he says. "Their bodies would dehydrate, their eyes would sink into their heads, their temples would become deeply sunken and they would turn a grey-green colour.

"It was miraculous that many were saved through doctors like Weary Dunlop. Many died a horrible death."

One of Uren's inspirations during his days in Changi was a letter from his mother containing the simple message: "Don't worry love, look for the silver lining".

It was advice Uren took to heart and he refused to allow bitterness towards his former captors to consume his post-war life, instead attempting to translate the lessons learned in the death camps into Australian political life.

"There's no progress in that type of hate," he says. "I don't hate the Japanese, I hate militarism and fascism," adding that during his incarceration and in the years immediately after the war he would have gladly seen the nation wiped off the face of the earth.

-- 'The dropping of those bombs was a crime against humanity' --

After the war, Uren attempted to revive his boxing career but his malaria-wracked body could not sustain the punishment and he retired after fighting a number of bouts in Britain.

Returning to Australia, he married and worked for a number of years as a grocery store manager before being elected as a Labor member of parliament in 1958. At the time, Labor was going through a long period of opposition and it was not until 1972 that he became a minister in the reformist government of Gough Whitlam, which withdrew Australian troops from Vietnam.

He was one of the world's first environment ministers, at a time when green issues were viewed as a fringe issue, and remained socially active in the peace and civil liberties movements.

After leaving parliament in 1987, Uren was voted onto Australia's inaugural list of National Living Treasures in 1997 for his public service but, well into his ninth decade, bridles at the suggestion he has eased into well-deserved retirement.

He remains an active supporter of East Timor's cause as the fledgling nation comes to terms with independence from Indonesia and campaigns for the rights of war veterans, as well as regularly visiting schools to spread his message of peace to a new generation.

Uren says his view on the use of nuclear bombs at Hiroshima and Nagasaki had changed since 1945, when he was elated they had ended the war and allowed him to finally return home.

"As I've grown to understand the outcome of nuclear arms and nuclear war and the nuclear industry, I believe that the dropping of those bombs on Nagasaki and Hiroshima was a crime against humanity," he says.

Despite the hardships he has endured and the horrors he witnessed, Uren remains upbeat about both human nature and his experience in life.

"I really can't complain," he says. "Life has been very generous to me."

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