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But in a decision that will further confound his critics, he will enter that contest in a bid to win his job back.
Soon after Cunliffe's announcement, Robertson also confirmed he will enter, and said the election result showed it was time for a new generation of leadership and change for Labour.
It is also possible the field will get bigger; David Shearer and Andrew Little are yet to rule out having a go themselves. Not all of Cunliffe's critics in caucus are fans of Robertson and some are pondering alternatives.
Image 1 of 13: Weekend Herald : Labour Leader David Cunliffe and wife Karen Price. 26 June 2014. New Zealand Herald Photograph by Richard Robinson. NZH 12Jul14 - Karen Price was one of David Cunliffe's helpers in his bids for the Labour leadership. Picture / Richard Robinson
The contest is unlikely to happen until the end of the year, with Labour's Council aiming to have a review of the election completed by the start of December and many in caucus want the review completed before the leadership is decided.
Image 1 of 11: the 3 standing for leader of the Labour Party - . Rod Emmerson 27/08/13 Editorial Cartoon WMM 27Aug13 - HBK 27Aug13 - NZH 27Aug13 -
Cunliffe was confident he still had strong support across the members and unions that handed him the job last year.
Robertson said this contest was a completely different ball game — despite having the same contestants.
He did not believe Cunliffe's prior resounding victory with the members and unionists would be repeated this time given the election results.
"This starts from scratch. A lot has happened in the last year and I'm confident I can get support across all three groups."
Image 1 of 7: Labour Party leader David Cunliffe speaks to his press secretary while on Herne Bay beach on Auckland's waterfront. 26 September 2014 New Zealand Herald Photograph by Peter Meecham
If Little opts against standing or is tipped out of Parliament on the final results of the election, his support could prove critical for one of the others because of his ability to deliver the union vote.
He is likely to side with Robertson if there are no other contenders.
One of Cunliffe's 2013 supporters, Iain Lees Galloway, would not comment when asked if he would support Cunliffe again, but said he was pleased there would be a contest for the role.
"I think the membership will appreciate being given the opportunity to have a say."
After Cunliffe's announcement yesterday, Labour Party president Moira Coatsworth said the party would decide by Thursday when that contest would be.
It will not formally be triggered until Cunliffe officially resigns at caucus on Tuesday.