Labour's deputy leader Annette King says it was her decision alone to step aside and retire from Parliament at the election.
Mrs King announced today she will also retire from Parliament at the September election, after more than 30 years in Parliament.
Labour leader Andrew Little is backing front-bencher Jacinda Ardern as Mrs King's replacement ahead of a caucus vote next Tuesday.
At a news conference Mr Little said Mrs King made her decision yesterday and then told him. He said he urged her to stay on in the health portfolio but she called him last night to say she was retiring.
Ms Ardern said she was 'humbled' by Mr Little's decision to nominate her.
Mr Little praised Mrs King.
"[She was] an absolutely outstanding deputy, loyal, hard working, full of incredible advice drawing on her vast experience."
Mr Little said Ms Ardern had fantastic networks within her generation which was suffering the most from the National government's policies on housing and health, among other issues.
He said he was not aware of anyone else putting their name forward for the role of deputy and Ms Ardern had strong support within caucus.
Ms Ardern said she would not take the vote for granted and would be speaking with her caucus colleges before the vote.
Time to 'pass on the baton' - King
She said she had been considering her position for some time, and after discussing the matter with her colleagues felt it was the right time to "pass on the baton".
"This is totally my decision.
"I have always acted in the best interests of the party and at the forefront of my mind is ensuring Labour is in the best possible position to change the Government in September."
Ms King was giving Jacinda Ardern her full support.
"I have watched her political career blossom since she became an MP in 2008 and mentored her when she needed help.
"After her emphatic victory in Mt Albert, she's well and truly ready to step up."
She denied she had come under any pressure to step aside.
"No, not from a single member of caucus and certainly not from the leader of the party...this is totally my decision.
"I went to Andrew [Little] and said that I wanted to step aside, that I also wanted to retire at the election, that the timing was right.
"If I'd wanted to stay there I'd still be there ... in fact when I said I'd stand aside Andrew was very keen for me to stay on and do health on the front bench. But actually it just feels right to me that this is the time to make the announcement."
Mr Little paid tribute to Ms King but said he respected her wish to retire.
"Annette has been a wonderful deputy since I became leader. She's tirelessly supported me and I've really appreciated her wise advice, humour and huge experience."
The caucus will hold a vote for a new deputy on Tuesday and Mr Little will nominate Ms Ardern for his deputy.
He said Ms Ardern had performed "extremely well" as a list MP and her "resounding" win in Mt Albert showed she had what it took to be deputy leader.
That prompted speculation at the time would replace Mrs King. Ms Ardern told Morning Report on Monday that was not something she had asked for, or discussed with the Labour leader Andrew Little.
Prime Minister Bill English said Mrs King would be missed by the Labour Party and that Ms Ardern was untested.
Mrs King announced late last year she would not stand in her Rongotai seat in Wellington, which she's held for almost 24 years. She said at the time she would instead contest the election as a list-only candidate.
Mrs King won the Rongotai seat with a 9600 majority in 2014.
Wellington deputy mayor Paul Eagle is to stand in the electorate.