Phil Goff and Annette King will officially resign as Labour Party leader and deputy in two weeks, triggering a race for the leadership of the party between three main contenders.
Mr Goff emerged shortly after 3pm on Tuesday from a five-hour meeting of the party's caucus to announce that he and Ms King would resign on 13 December.
He had earlier said he accepted responsibility for the party's resounding election defeat but wanted to talk to his caucus before making any public announcement.
Five MPs have put their names forward as either leader or deputy. David Cunliffe, David Parker and David Shearer, who are seen as the main contenders, and Grant Robertson and Nanaia Mahuta.
Mr Goff said other candidates may yet come forward in the next few weeks, and that he would not publicly indicate a preference for any one individual before he cast his vote in caucus.
"I want to pledge whoever might be leader or deputy leader of the Labour Party that they will have my full support and active support from whichever position I hold in the House, which will not be a front-bench position."
Labour's share of the vote on Saturday dropped to 27% from 34% in the election three years earlier. It was the party's worst result under the MMP electoral system.
Race to lead party
David Cunliffe confirmed that he would contest the leadership with Hauraki-Waikato MP Nanaia Mahuta standing as candidate for his deputy.
Mr Cunliffe told Radio New Zealand's Checkpoint programme there was no foundation to criticism that he failed to give Mr Goff enough backing, in particular on policy costings which the Labour leader stumbled over during the campaign.
"I think it's pretty obvious that someone's tried to have a dig before the race has even started," Mr Cunliffe said.
"The tax numbers that Phil was questioned about were in the public domain a long time before that debate and I certainly wasn't asked for any briefing on that day that I did not provide."
He said the party wants to reach out to people who think it had some good ideas and policies but aren't yet buying the brand.
Mr Cunliffe said he and Nanaia Mahuta would have broad appeal including to Maori, Pacific, working Pakeha and people of all ethnicities.
David Parker, the party's economic spokesperson, is Mr Cunliffe's main rival and is expected to have Wellington Central MP Grant Robertson as his running mate.
He told Checkpoint that in the past Labour had fought and won victories for the working class and now needs to be relevant to today's working people.
He wants to see the Labour values of hard work and equal opportunity reinforced around New Zealand and thinks the National Government is taking the country in a different direction.
He said he was more than capable of being leader and it was time to let his light shine.
Mount Albert MP David Shearer, who entered Parliament in a 2009 by-election, said he was new and fresh and had experience he could bring to the Labour Party.
"I've built teams before, sometimes in pretty difficult situations. I think that I offer something."
List MP Shane Jones, who has ruled himself out of the leadership contest, said before Tuesday's caucus that he didn't expect to hear too much from the contenders during the meeting.
"We've got to delve deeply into why three out of every four New Zealanders who cast a vote said we're unsound and unfit to govern," he said.
'History will be kinder'
Assistant professor of politics at Auckland University Raymond Miller said historians will be much kinder than current commentators have been about Phil Goff.
"I can't imagine that anyone within the Labour caucus could have done a better job in an election where it was obvious that the voters didn't want change."
Professor Miller told Morning Report that Labour was always going to lose because, having voted for new government at the previous election, voters did not want another new administration.
He said Labour needs to rebuild itself comprehensively and bring in fresh people - a process he said should have started in 2005 after six years in power.
For instance, he said, of the top 25 members of the Labour Party list in the 2011 election, only one was a non-MP, and that was Andrew Little who's a former party president. He said the party needs to attract a new base of support.