John Key says he made the decision to step down as Prime Minister not long after telling media he intended to stand for a fourth term.
Mr Key was questioned about his future as National Party leader while in New York supporting Helen Clark's ultimately unsuccessful bid to become United Nations Secretary General in September.
When asked by The Nation if he'd see out a fourth term in Government, Mr Key said: "Yeah, that's my intention...but the first point I'd make to you is you've got to win a fourth term."
He told Newshub's political editor Patrick Gower it "wasn't [his] intention" for Kiwis to vote for him, only to then step down after securing a fourth term.
But Mr Key has now revealed he was already considering resigning at the time.
On Monday, he told Duncan Garner on RadioLIVE he spoke to Deputy Prime Minister Bill English when he returned from New York and told him he was "99 percent sure [he] was going to transition out of the job at the end of the year".
Mr Key said he and his wife Bronagh had already been discussing whether he should stay or go while they were on holiday in Hawaii earlier in the year.
"During the year we were mulling it over a little bit, we went to Hawaii in the middle of the year and had quite a long chat about it," he said.
"I really thought this was probably the right call [as] the end of the year was going to mark my decade as being the [National Party] leader and eight years as Prime Minister."
At his resignation announcement on Monday, Mr Key says he recalled driving home from media interviews with Bronagh as they discussed his realisation that he needed to resign.
"You guys did all ask me... whether I'd absolutely stand for a full fourth term, and one of the things I said to Bronagh on the way home was look I don't feel comfortable looking down the barrel of the camera and not being honest," Mr Key said.
"When we went off to the Security Council and came back in September I really just thought, 'Wow it's been a remarkable ride but I think that's about it'."