Dr Cullen, 76, announced in March last year thathe had been diagnosed with stage 4 small-cell lung cancer, which had also spread to his liver.
Born in London in 1945, Cullen moved to Christchurch with his family when he was 10 and was educated at Christ's College, where he was a scholarship boy, and at Canterbury University. He studied mathematics for his first degree, moving to history for his MA. Later, he took a doctorate in economic history at Edinburgh University.
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.
A full obituary will follow.
