Labour MP David Cunliffe said he gave it his best shot in politics, but is happy to be bowing out.
Mr Cunliffe told the Paul Henry programme he departure came after he was given a new job opportunity that was "too good not to say yes to".
Mr Cunliffe will join the leadership team at management consultancy firm Stakeholder Strategies Limited in Auckland, after seeing out his term as New Lynn MP.
Commenting on Labour's defeat in the 2014 general election - the party's worst since 1922 - Mr Cunliffe said he may have misjudged the leadership role against the political climate of the time.
"I did perhaps underestimate all of the work and all of the resource requirements of turning a party around one year out from a general election," he said.
"Nobody could have predicted the weirdness of the Dotcom phenomenon, that German billionaire wrecking ball that swung through New Zealand politics and became a one-man turnout machine for moderate National voters."
Mr Cunliffe said he is "quite determined" that Labour will win the 2017 election.
"Our caucus is in great shape," he said.
"The left block, Labour and the Greens, on the last three major polls have all been ahead of National and its allies... There is at least an even chance of a change of Government."
Mr Cunliffe said New Zealand had not been served well by the Dirty Politics controversy, and added he hoped Kiwis would "really reflect on what we want" in the 2017 election.
"You look at American politics, and we see a very vitriolic, very polarised community.
"We have so much more in terms of strong, shared values about a fair go, everybody gets a shot, let's make it the best country we can," he said.