Exiting Labour Party MP Shane Jones says he no longer had the level of commitment or energy that his party deserved.
Mr Jones said he had told Labour leader David Cunliffe of his decision as well as the Party President Moira Coatsworth.
He has been in talks with Foreign Minister Murray McCully for some time about a possible role in an international fisheries management role and expected that to go through.
He said he would leave at the end of next month after reflecting over Easter about his future.
On TVNZ's Breakfast show today, Mr Jones said he would not be returning to the political sector.
"I've made my decision and I'm moving on."
There was never any pressure on him to leave, he said.
It was not fair to say that with his departure would make it harder for Labour to win this year's election, Mr Jones said.
"The voters have got to make the call on what sort of Government they want in the future.
He said he was "flattered" when he was contacted by Mr McCully about the Fisheries role.
"The initial discussion was some time ago and one discussion earlier this year between Murray McCully and myself.
"But I have to say I've got to clear the decks first, I've got to be honest with David Cunliffe and David Parker and President Coatsworth that I no longer have the commitment or energy to be a credible member of the team - so that's it."
Mr Jones told TV3's Firstline that the National party had not poached him.
"There's no signed and sealed deal on the table, I just took time out over Easter, I'm 55 this year and was honest with myself - I no longer have the energy and commitment to bring to the role of being a front bench fighter for the Labour party, both coming into the election and for another three to six years.
"The party deserves to be represented by people who have 1000 per cent commitment."
He said he gave the Labour leadership bid his best shot last year, but much to his disappointment was a "bronze medallist".
Mr Jones said he had worked hard to expose Countdown's alleged mistreatment of New Zealand suppliers and the issue had now been passed on to Labour's commerce spokesman Clayton Cosgrove.
"He is already developing a policy paper to rein in the excesses of the supermarkets."
Mr Jones said he was leaving the Labour party so close to a general election because he had to be true to himself.
"The honest truth is; the ebb and flow of Labour's fortunes have at the margin, a bit to do with me, but at a deeper level it's to do with winning the hearts and minds of Kiwis - to do that they need people that are dedicated an can commit 1000 per cent and I just admitted to myself that's not the space I'm in.
"Two or three years ago a number of us Labour MPs went up into the Pacific on a delegation with business people and I reflected a long time ago that if I ever did move on from politics that's something I'd like to do.
"I came from the sector - fishing and Maori politics - and I've always felt I would have something to offer."
Mr Jones said he hoped Kelvin Davis, who he brought into parliament in 2008, would replace him.
"I am confident that he will be a credible voice. He's got a good level of credibility in the area of education.
"I know that I have disappointed one or three people in respect of my decision, but I've got to say, if you want to put your hand up and secure the privilege of representing New Zealand, you first have to be truthful to yourself, and the truth of the matter is, I no longer have the passion, commitment and energy to credibly represent the Labour party."