Mr Jones told the Herald his unsuccessful bid to become leader last year was part of his decision, but he also wanted to try a new opportunity.
"Over Easter I just had a deeply introspective time and was truthful to myself. I don't want to do it any more. The leadership had a bit to do with it, but I'm 55 this year and I just want to go and do something else. I'm not able to give to Labour the 100 per cent that I ought to be giving. They need a team to stay for the long term and I'm not in that space."
On TVNZ's Breakfast show this morning 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.
There were frustrations during his career in being reigned in over some comments he had made, Mr Jones said.
"The political collar has chafed this dog's neck and now I've slipped the collar.''
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.
Mr Jones' resignation means Kelvin Davis, Labour's candidate in Te Tai Tokerau, is likely to return to Parliament for the few months before the election.
"I am confident that he will be a credible voice. He's got a good level of credibility in the area of education.
Close friends said it was possible other reasons contributed to Mr Jones' decision, including a realisation he would not become leader, some recent periods of ill-health and some disgruntlement at being told to rein in his public appearances and being reprimanded for attacking the Greens.
Reaction within Labour
There has long been speculation Mr Jones would leave after the election if Labour was still in Opposition, but his decision to leave earlier clearly caught the party by surprise.
Labour leader David Cunliffe said Mr Jones would be missed. He said the MP had made it clear that his decision was nothing to do with either Mr Cunliffe or the Labour Party.
"He's made an important decision for his own reasons and we respect that and wish him well."
Mr Jones told Radio New Zealand Mr Cunliffe was disappointed in the decision but accepted it.
While Mr Jones spoke to a constituency many in Labour could not reach, Mr Cunliffe said there were others within Labour who could fill that gap.
"We will make sure we continue to work for all Kiwis, including those who have an ear for the korero Shane has been giving."
Former Labour leader David Shearer said the decision had taken him by surprise. "I'm really sad. He is a good mate and he was showing in the last few months just how great a politician he is and what he could be."
Other MPs said they were surprised by the timing, given Mr Jones' success as Labour's impact player over the past few months, building on his performance in last year's leadership challenge by forcing a Commerce Commission investigation into the treatment by Countdown of New Zealand suppliers.
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.''
McCully: Jones shoulder-tapped for role
Murray McCully confirmed he had shoulder-tapped Mr Jones this year to ask him to consider taking a new role working on New Zealand's economic development aid programme across the Pacific and in other small developing countries.
"There's no question he is the best candidate we could appoint to this role and I've made that clear to him."
He knew Mr Jones well from frequent delegations to the Pacific and said his experience on the Maori Fisheries Commission and commercial background fitted the role perfectly. He said Mr Jones would also play a role in New Zealand's bid for the Security Council and the Small Island Developing Nations meeting in Samoa this year.
Mr McCully denied the role offered to Mr Jones was politically motivated and the Labour MP was the best man for the job.
The position needed a "complex mix'' of commercial skills and political leadership skills to try to work with different governments.
The role was not created with Mr Jones in mind, he told RNZ.
"We wanted to appointment someone for the role, if he didn't want it then obviously we would need to appoint somebody else.''
Mr McCully said he was sure there would be some cynicism about the job offer.
"Shane Jones makes his political decisions for himself. I simply inquired as to whether this was the timing where he was thinking about making a move.
"I hope that he'll take up the job, I hope that what we've seen yesterday is a in the nature of a clearing of the decks so that he can seriously discuss a role that is both important to this country and to the region and that is something that can only be concluded after he'd made a personal and political decision.
Prime Minister John Key had known for "some weeks'' that Mr McCully would be speaking with Mr Jones about the Fisheries role.
Jones: I'm not a sellout
Mr Jones denied being a "sellout'' in leaving for a role offered by the National party.
"Being in politics is also like being at a holiday inn, you serve your time and then the next person arrives to take your place,'' he told Radio New Zealand this morning.
Winston Peters: Jones' departure "major loss" to Labour
New Zealand First leader Winston Peters told RNZ it was a "turn up for the books'' that Mr Jones would take up a job offer that had come from the National Party.
"Last year, year five of National, they underspent their foreign aid budget by $92 million, so if they were so concerned about the economic development of the Pacific, then why were these massive cuts taking place in order to try to obtain a budget surplus?''
He said Mr Jones needed a "long contract'' that he was willing to help with.
"I hope that Shane has got this in writing.''
Mr Jones' departure would be a "loss to Parliament, it'll be a major blow to the Labour Party and an even bigger blow to Maoridom'', Mr Peters said.
It was possible Mr Jones would return to politics in the future, he said.
"If you've got real ability you can do it, and he has, there's no doubt about that.''
Bit it might be a "bridge too far'' for him to return to the Labour Party, Mr Peters said.
Dover Samuels, a former MP and close friend to Mr Jones, said the Labour Party should take some of the blame for failing to keep him.
"He always pointed out to the Labour Party that if you didn't take middle New Zealand with you you will be in the Siberian ring of the Opposition for the rest of your life. And I think, sadly, they didn't hear that. They've got their own agendas." He said Labour had lost its chance to support the first potential Maori Prime Minister.
Despite his successes, Jones' political career was blighted by controversy. In 2009, the release of ministerial credit card details showed he had been using his to watch porn in hotel rooms although he had repaid the amounts in question.
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- Additional reporting Brendan Manning, Rebecca Quilliam of APNZ
