Cunliffe was flanked at his announcement by MPs Nanaia Mahuta and Iain Lees-Galloway, who signed his nomination form, along with other MPs including Louisa Wall and Carol Beaumont.
Asked how he would take Labour to victory in 2014, he said today was the start of the road to earn the confidence of the Labour party and the wider labour movement. "Like a bundle of reeds we will be unbreakable," he said.
"John Key and his forces of conservatism are going to find a whole new fighting machine up against them."
His message to John Key was to "book a very long holiday in Hawaii".
Mr Cunliffe said his first duty on taking over the leadership would be to meet each member of the labour caucus, making sure they were a combined force that could win over the country.
He said he was a different man than he was a year ago, when he was demoted by David Shearer over his leadership aspirations.
"I certainly had to learn a few lessons and I think my colleagues would all say, all of them, that I have, after a few bumps in the road, knuckled down and done nothing but work collectively for the team."
He would not be drawn on who he would want as his deputy, but said that Grant Robertson would have a senior place in his team.
"I'll be happy to have him in my senior team and I think the reverse is true as well.
"The great thing for Labour is that whatever the outcome we're going to be in strong shape."
When questioned about living in one of Auckland's most exclusive streets - in Herne Bay - outside his electorate, he said he had moved out of the area when he had a young family, so his wife, who was a Queen St environmental lawyer, could breastfeed.
"We were lucky as a couple of kids that came from very modest backgrounds we were able to work hard and get ahead. The point is that Labour people don't pull up the ladder when they've done it, they want the ladder to be there for everybody else."
Asked whether he would raise taxes, he said he would not pre-empt Labour's policy process, but that people on wages and salaries should not subsidise people who earned millions of dollars.
He said Green Party co-leader Russel Norman would be unlikely to be Finance Minister under a Cunliffe-led Labour government, but he looked forward to working more closely with both Mr Norman and fellow co-leader Metiria Turei.
