UPDATED 10.27am Labour will elect a new deputy leader after Annette King decided to step down and retire from Parliament, leader Andrew Little has announced.

LISTEN ABOVE: Annette King spoke to Leighton Smith after announcing her retirement from politics

Annette King told Leighton Smith the decision is all her own... and she's had nothing but support from the Labour caucus.

"I've been there 30 years. I've been through 10 elections, eight leaders and I can now look and see that the caucus has got some fantastic talent. You look at some of our candidates that we're lining up."

“It’s a decision I’ve made... I’ve achieved what I’ve wanted to achieve. I feel I’ve done my work,” she said.

"The time is right. I've been working for the past six to ensure that I've got a really good replacement for me in Rongotai, and two weeks ago Paul Eagle was confirmed as our candidate."

She said becoming of Minister of Health had been a highlight of her political career.

“I really loved that role, probably because I came from a health background.”

Labour leader Andrew Little said: “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”.

“She’s also been an excellent Health Spokesperson. While I wanted her to stay in that role, I accept her reasons for wanting to retire. She has been an outstanding servant of the Labour Party and worked hard for a better and fairer New Zealand. She has made a huge contribution in government, opposition and in Rongotai.

Thanks to @annetterongotai for her incredible support, friendship, humour and counsel. Over 30 years service to NZ: a true Labour hero. pic.twitter.com/WmHNjlLkom

— Andrew Little (@AndrewLittleMP) February 28, 2017

King praised Little saying he remained the right for the top job.

“The more people that meet him the more they will see they have a man of substance as prime minister.”

Labour MPs will elect the new deputy next Tuesday. Little will nominate Jacinda Ardern for the deputy position.

“She has performed extremely well as a list MP and her resounding win in the Mt Albert by-election is further proof that she has what it takes to be my deputy.”

Today’s announcement comes after speculation Ardern’s showing in the weekend’s Mt Albert byelection could see her elevated to the deputy position ahead of September’s general election.

King told Leighton Smith she would be supporting the nomination of Ardern to fill her deputy role.

“I have been a friend and mentor to her since 2008 and I’ve watched her grow.

“She will be a very good deputy.

“She’s ready.”

It follows a week in which King vehemently rejected any suggestion she would be replaced as deputy by Ardern, saying suggestions it should happen were 'ageist.'

Ardern's victory in the Mt Albert byelection had re-ignited talk about whether she should replace King as deputy leader because of her Auckland base and high profile and to counter the change in National's leadership team to Bill English and Paula Bennett.

Little had said after the byelection there was "no vacancy" for the deputy slot and he wasn't "planning any changes".

Despite that in her statement, King said the decision to stand down was hers alone - any move to unseat King against her wishes would upset many in caucus, where King commands a great deal of respect and influence. 

Many MPs believed she should remain in the role where she was described as the "glue" that holds caucus together, alternating between being a supportive aunty to a Principal reprimanding MPs who go astray.

The deputy role is chosen by a caucus vote but the leader's preference should have sway.

Labour MPs are reacting to the news.

Stuart Nash, the party’s police spokesman, said King was “absolutely fantastic” and would be missed by her colleagues.

“But good on her for making this decision. She’s going to be 70-years-old at the next election. The love of her life is her little grandson William, and I take my hat off to her. I love Annette dearly.”

King’s decision was made for the good of the party, Nash said, and that reflected on her character.

“She is a woman of true integrity and she has always been 100 per cent loyal. She has always had my support and whoever replaces her will equally have my support.”

Corrections spokesman Kelvin Davis said the first thing was to pay tribute to King, and the work she had done for New Zealand and Labour.

“And wish the new regime - whoever that may be - the best of luck. It is exciting times ahead for us.”

Grant Robertson wrote of King on Twitter that “there is no one I know who has given more to the pursuit of Labour values of social justice”.

There is no one I know who has given more to the pursuit of Labour values of social justice than @annetterongotai Thanks my friend.

— Grant Robertson (@grantrobertson1) February 28, 2017

Wellington's mayor has thanked Annette King for her enormous contribution to the city over her career.

Justin Lester has tweeted a message, saying' Thank you Annette, you're an outstanding woman'.

.@annetterongotai has made an enormous contribution to Wellington and NZ over her career. Thank you Annette, you're an outstanding woman.

— Justin Lester (@justin_lester) February 28, 2017

King was initially appointed as an interim deputy for Little to provide experience and a guiding hand as he settled into the role of leader in December 2014.  

He had expected to appoint a new deputy when he did a reshuffle in October 2015.

Instead he opted to keep King on until the election. At the time, Little said Ardern told him she did not want the role: "The nature of the deputy's role is there's a lot of back office stuff that has to be done and that's been very important. Jacinda's strength is the outreach and getting out there, especially in Auckland where I need her to be most active."

Ardern said she had not put her hand up because she believed King should remain in the role and her own focus was on trying to win the Auckland Central electorate in 2017.

That would have been her third attempt to get the electorate from National's Nikki Kaye, but Ardern's Mt Albert byelection win has taken that out of the equation.