Sir Michael Cullen died last night in Whakatāne. He was 76.
In March 2020, Cullen said he had been diagnosed with advanced lung cancer.
Former Prime Minister Helen Clark, whom Cullen served with as Minister of Finance and Deputy Prime Minister, said she received the news with "profound sadness".
"Michael was an utterly dependable Deputy Prime Minister and Minister of Finance. One could throw any big and knotty policy challenge at him - there were many of those, and he would find a solution for it.
"Our Government was indebted to his courage, determination, insights, and brilliance," she said.
Clark said Cullen had a "deep commitment to social democratic values".
"He was deeply committed to making New Zealand a place of which everyone could feel proud and in which each could feel that they had a personal stake," she said.
Clark said as well as the Superannuation Fund and KiwiSaver, Cullen's legacy included interest-free student loans, 20 hours' free early childhood education, more affordable primary health care, the establishment of Kiwibank, and buying back Air New Zealand and KiwiRail.
Prime Minister Jacinda Ardern said New Zealand was "richer, in every sense of the word, for Michael's life".
"He gave his life to making this place better for everyone," she said.
"He fought for social justice at every turn, understanding the need for balance and pragmatism at times, but always focused on the big picture and the long term."
Former Prime Minister, and Cullen's old political opponent, Sir John Key said he "had enormous respect" for Cullen.
"He was academically brilliant and extremely funny even when much of the time I was the target of that humour.
"His legacy in the area of savings, both KiwiSaver and the Superfund, will be enduring.
"History will remember him well, which is maybe the ultimate accolade for a professionally trained historian."
Former Deputy Prime Minister Winston Peters, who served in Parliament and Government with Cullen over much of his career, said Cullen "quickly established himself as an intelligent, well-educated and witty Parliamentarian, which is not common".
"He made a difference as Deputy Prime Minister at a very difficult time," Peters said.
"He was very much like, in different ways a combination of David Lange and - dare I say it - [Robert] Muldoon in terms of being capable of being understood."
Deputy Prime Minister Grant Robertson said Cullen had made an "enormous" contribution to New Zealand's long-term economic prosperity and stability.
Cullen anointed Robertson his "successor" in his recent memoir, Labour Saving.
"His contribution to New Zealand's long-term economic prosperity and stability has been enormous. As the architect of KiwiSaver, the New Zealand Super Fund and Working For Families he has left behind an economically more secure country," Robertson said.
"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."
Robertson told RNZ Cullen had been a "rock" of support and kindness after he took the role of Finance Minister in 2017.
One of Cullen's closest former colleagues, former Health Minister Annette King, said Cullen was "gone far too early - he will be sadly missed".
King got to know Cullen when she entered Parliament in 1984, one term after him.
"In 1984, I was a rookie and he was already established.
"I first saw his formidable memory and wit when he invited me to one of his famous BNZ debates.
"Michael Laws organised them. [Cullen] invited me to be on his team in these BNZ debates and I saw this incredible wit and intellect and the fastness of his repartee," she said.
King recalled Cullen's wit could quickly defuse tense Labour caucus meetings.
"On many occasions when there could be tense conversations in the caucus, one of Michael's one-liners would dissolve the caucus into laughter," she said.
Cullen had the "endearing quality" of passing notes to his colleagues on subjects he thought they would find amusing.
"I don't think I can probably share them," Kind said.
"They were personal notes but they were always on target and something silly that would happen," she said.
King said the Clark Government relied on Cullen as a problem solver.
She said she never walked out of a meeting with Cullen unhappy.
"[Associate Finance Minister Trevor Mallard] was always the hard man in negotiations around Budget time. Michael would listen to the argument I'd put up wanting something for Health and then come up with a solution," King said.
In later years, Cullen flatted with King.
King, who is currently serving as High Commissioner to Australia, said she had Zoomed Cullen often in the past year.
Her last call with him was three weeks ago.
Cullen was first elected in 1981 as MP for St Kilda and made deputy Labour leader in 1996. He was Deputy Prime Minister to Helen Clark for six years from 2002, and he served for three terms as Finance Minister from 1999 to 2008.
Clark wrote in the foreword to Labour Saving that Cullen was one of New Zealand's greatest finance ministers, had a keen intellect and could turn his hand to almost any task.
She said the book highlighted his role as "a thinker, doer and builder in a long-standing Labour social democratic tradition".
He had a famously quick and acerbic wit and thrived in Parliament's debating chamber.
Cullen retired from politics in 2009, after Labour was defeated by John Key's National Party at the 2008 election.
Cullen was put to use by both National and Labour governments after leaving politics – as chairman of NZ Post, reviewing the intelligence agencies, and leading the Tax Working Group which recommended a capital gains tax.
He was knighted in the 2012 Queen's Birthday Honours.
Cullen was responsible for setting up the Superannuation Fund, dubbed the Cullen fund, which is designed to smooth out the costs of the state-provided pension for the Baby Boomer bulge. It currently stands at $58 billion.
He also established the KiwiSaver retirement savings scheme to which three million people belong.
And he introduced the Working for Families package for payments and tax credits for families with dependent children.
He was put in charge of responding to the 2003 Court of Appeal decision on the foreshore and seabed, which opened up the possibility of the Māori Land Court awarding freehold title in the foreshore and seabed.
He was also Social Welfare Minister in the Fourth Labour Government which split over Roger Douglas' reform agenda.
Robertson paid tributes to Cullen's work with Labour during the Douglas split, telling RNZ Cullen stuck with the Labour party "at the toughest of times in the 1980s".
Cullen delivered nine Budgets from 2000 to 2008, and he claimed to have delivered nine surpluses. But the 2008–2009 year, which included part of the financial global crisis, ended in deficit and was effectively shared with his National successor, Bill English.
Former political adversaries also mourned Cullen's loss.
National Leader Judith Collins, who was in Opposition while Cullen was Finance Minister, said: "Sir Michael was a consummate politician – a respected adversary and a man of absolute conviction.
"But, most importantly, he was a man of uncommon and uncompromising principle."
She noted that National had called on Cullen after his retirement from politics to chair NZ 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," Collins said.
Former National Cabinet minister Steven Joyce tweeted that while he and Cullen were "clearly not on the same side of politics" he had enjoyed working with Cullen in his retirement.
"An unstinting servant of NZ," Joyce said.
Cullen was always a strong supporter of socially liberal issues including homosexual law reform in 1986, and more recently he publicly advocated for David Seymour's End of Life Choice Bill.
He and his first wife, Rowena, had two daughters. His second marriage was to Anne Collins, a former Labour MP.
Cullen said in his book Labour Saving that he was not fighting a battle against his cancer.
"It will do what it will and, in the meantime, I will do what I can. Death is no more than the space we make for others to live."
Re-styling the Dylan Thomas poem, Cullen said: "I do wish to go gentle into that good night. Raging against the dying of the light is a pointless exercise, certainly after a reasonably long and very fulfilling life."
He urged New Zealanders to vote "yes" to euthanasia in the successful 2020 referendum.
"It does not force any medical professional against their conscience," Cullen said. "It respects the rights of those who find assisted euthanasia morally abhorrent.
"But it offers to people like me the chance of finishing the life I have enjoyed so much in a way consistent with my moral beliefs and my sense of the dignity of human life."