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Dr Cullen, 76, announced in March last year that he had been diagnosed with stage 4 small-cell lung cancer, which had also spread to his liver.
London-born, Dr Cullen's family moved to New Zealand when he was 10.
After graduating from the University of Canterbury he became a lecturer at the University of Otago, and became an increasingly more important figure in Labour Party politics.
After serving on the party's executive council he was elected MP for St Kilda in 1981.
He was later MP for the renamed Dunedin South seat, before becoming a list MP.
When David Lange became Prime Minister in 1984 Dr Cullen became chief whip, before entering Cabinet in 1987 as associate finance minister and minister of social welfare.
When Labour returned to the Opposition benches he became finance spokesman and was a ringleader in a failed attempt to convince party leader Helen Clark to step down.
When Miss Clark was elected Prime Minister in 1999 Dr Cullen was her loyal deputy and a leading figure during her nine years in power.
Shortly after Labour lost the 2008 election Dr Cullen retired from politics, but went on to hold several prominent roles, including being chairman of the Tax Working Group instigated by Jacinda Ardern.
'He wanted to make a difference' - Helen Clark
Former prime minister Helen Clark worked with Sir Michael for 40 years and told RNZ his death was not unexpected but it was very very sad news.
"He was an utterly dependable deputy Prime Minister and Minister of Finance. You could throw any big and knotty problem at him, he would find a solution for it," she said.
"My thoughts are with Anne Collins - Michael's partner - and his family. This is not unexpected but it leaves a huge gap in their lives, and a huge gap in our country's life given the very substantial contribution that he made.
"Without his courage, determination, insights - brilliance, really - we would not have been able to do many of the things that we were able to do as a government."
She said he came from humble origins and was a big-picture thinker who wanted to make a difference, to better people's lives.
"He was deeply committed to making New Zealand a place that everyone could feel proud of and feel they had a stake in. A very committed social democrat - that comes through strongly in his book - I think that was what drove him.
"I think when we remember Michael we'll remember the public face which was the witty, the clever, the quick on his face in Parliament. We should also remember that he was a shy person, he lived a very modest lifestyle, actually quite a humble person and I think the greatest tribute we can make to him is that he made a difference for the better.
"If each of us did that, our world would be a better place."
'Quick wit, great debater'
Auckland Mayor Phil Goff entered Parliament with Sir Michael in 1981 and spent more than 30 years working with him, and said he was one of the sharpest minds there.
"It's really sad news ... his quick wit, he was a great debater," he said.
"He was consistent, he worked hard, he was on top of his portfolios - particularly the finance portfolio - and he was one of the strengths, I think, of the Clark Labour government."
The legacy of Sir Michael was fantastic and left New Zealand in great shape financially to be able to handle the global financial crisis and the Christchurch earthquake, Goff said.
"He was not only the minister of finance ... he was also a strategic leader and worked closely with Helen and the rest of us... If there was a problem and somebody needed to be pulled out of their normal portfolio responsibilities to focus on that problem and find a solution to it, Michael was the person that Helen [Clark] would go to.
'One of the most influential figures'
Prime Minister Jacinda Ardern and Finance Minister Grant Robertson said in a statement the government was mourning Sir Michael's death.
"New Zealand is so much the richer, in every sense of the word, for Michael's life. He gave his life to making this place better for everyone," Ardern said.
"He was a great friend to me and to many of the Cabinet and Labour caucus. We will miss him terribly, and we are sending all our love to Anne and his family."
"Sir Michael was one of the most influential figures in New Zealand politics over the last 40 years. Intelligent, funny and kind he left a significant legacy for the country."
Robertson said Sir Michael's contribution to New Zealand's long-term economic prosperity - as the architect of KiwiSaver, NZ Super Fund and Working for Families - was enormous.
"Each of these policies on their own would define a political career, but taken together they represent one of the most significant contributions any politician has made in recent times," he said, "For Michael they are only part of his extraordinary contribution to New Zealand."
He said he would also remember Sir Michael for small acts of kindness.
"He was always ready with a piece of advice, an insight or just a funny story. He also helped many Labour MPs out, often behind the scenes, and not wanting any publicity.
"He was loyal to the Labour Party, but also not afraid to voice his concerns."
'A respected adversary'
In a statement, National leader Judith Collins said Sir Michael was a consummate politician, a respected adversary and a man of absolute conviction.
"Most importantly, he was a man of uncommon and uncompromising principle," she said.
"When he retired from politics in 2009, such was our respect for him we called on his vast skills and knowledge to chair New Zealand Post and Kiwibank, as well as appointing him to lead a wide-ranging review of New Zealand's intelligence agencies.
"He will be remembered as one of our most effective ministers of finance, with a long-term view of what needed to be done to enhance to New Zealand's economic and social prosperity and stability. His passion was to make New Zealand a better place for everyone.
"Sir Michael's keen intellect, wit and vast expertise will be missed by us all, and our thoughts and condolences are with his family."
In a tweet, she said she was saddened to learn of his passing, and the party extended deepest condolences to his family while acknowledging his many years of service.
A full obituary will follow.
- additional reporting RNZ