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

"It's a decision I've made. I've achieved what I've wanted to achieve. I feel I've done my work," King told Newstalk ZB's Leighton Smith this morning.

Andrew Little said he would be nominating Jacinda Ardern, newly elected to the Mt Albert seat, as his new deputy when the caucus meets next Tuesday.

Labour Party press conference.

"Annette has been a wonderful deputy since I became leader," Little said. "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."

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Speaking to media at Labour's Auckland office, Little said Ardern was "what we need right now", and he wasn't aware of others who were interested in the deputy role.

King's decision was "entirely hers", Little said.

"There is a vacancy now, it has to be filled, and I'm backing Jacinda."

Little said King told him yesterday of her decision. Asked if he had asked King to stay on as deputy, he said he had asked her to stay on the frontbench and as health spokeswoman, but she had decided to retire.

Ardern said King called her yesterday to tell of her decision.

"Anybody who knows Annette well knows that she knows her own mind. I felt very humbled that she suggested she'd like me to take on the role.

"I've always said it's not a job I would ask but if I was asked then I would happily take on the role. I believe I have what it takes and we have a strong team to take on the Government."

Asked if Ardern would have "100 per cent support" from Labour MPs, Little said he anticipated strong support.

"I am going to put morning tea on if that helps," Ardern added.

Little said the line-up was presenting "the fresh face of the next Labour government".

"Annette has been a staunch campaigner on health but she has made her decision about her political career. As leader I have to make sure we have the team in place to lead us to victory in September."

King said becoming Minister of Health, a role she held from 1999-2005, was the highlight of her political career.

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

As health spokeswoman, King's exchanges with Health Minister Jonathan Coleman became a feature of Parliament's question time.

Today, Coleman challenged to Ardern, tweeting her to say if she "really backs herself" then she would demand the health portfolio.

Coleman also said he was sorry to see King - "a worthy opponent" - go, and that she deserved better from "panicked Labour".

King said with Paul Eagle confirmed as Rongotai's candidate the timing was right to step down.

"This is a decision I made and I've been working up to it for some time."

She was bowing out of politics with the party renewed and revitalised with fantastic talent in the caucus ranks.

The party had performed well in recent byelections and this boded well for the upcoming general election, she said.

The resignation capped off a fulfilling three decades in Parliament.

"I've had 10 years' Cabinet experience and many people never get the chance to have any so I've had a really lucky career."

King also supported Ardern to replace her.

"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."

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

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

Reacting to the news, Labour MP
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."

Labour finance spokesman Grant Robertson tweeted that "there is no one I know who has given more to the pursuit of Labour values of social justice".

Corrections spokesman Kelvin Davis said the first thing he wanted to do 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."

Papakura MP and Minister of Energy Judith Collins, who shared a slot with King on the Paul Henry show, said she felt sorry for King.

"She has given everything for the Labour Party and you couldn't find a more loyal, hard-working, interested and, frankly, energetic deputy in that role. But, you know, politics is a brutal business. I feel quite sorry for her."

Asked if it was the right call to bring in Ardern, Collins said it was up to Labour to decide.

"I think Annette will no doubt continue to support them and do what they can."

Collins said King was a tough proposition if she was on the other side of your portfolio, but well liked across Parliament.

"She has been around a long time, but what is interesting with her is she still has the fire in her belly and has the work ethic and energy. Anyway, it's up to [Labour]."

Little, nominating Ardern as his deputy, said she had performed "extremely well" as a list MP and her recent win in the Mt Albert byelection was "further proof that she has what it takes" for the role.

Today's announcement comes after speculation Ardern's showing in the Mt Albert byelection would see her elevated to the deputy position before September's general election.

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

King said the decision to stand down was hers alone.

Any move to unseat her 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 aunt 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.

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 this year's general election.

It 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.

JACINDA ARDERN
• 36 years old, grew up in Morrinsville, after early years in Murupara.
• Has been a list MP since 2008, narrowly losing twice in Auckland Central against National's Nikki Kaye.
• Recently moved into Mt Albert with her partner Clarke Gayford, and is now the MP for Mt Albert after winning Saturday's byelection.
• Despite being marked as a future leader, has never stood for the Labour leadership in the three contests since 2011, but was on Grant Robertson's ticket following the 2011 general election and he had anointed her as his deputy.
• Her party's spokeswoman for justice, arts, culture and heritage, children and small business.
• Highly rated in the Herald's Mood of Boardroom survey of CEO's, is also adept at "soft" media - appearing on the cover of Next magazine and featuring in women's magazine stories - and even played a 45-minute DJ set at Auckland's Laneway festival in 2014.

ANNETTE KING
• 69 years old, grew up in Murchison in Tasman. After training and working as a school dental nurse, King joined the Labour Party in 1972.
• Elected to Parliament as MP for Horowhenua shortly after Sir Robert Muldoon called the 1984 snap election. Now the long-serving MP for Rongotai in Wellington.
• Was Health Minister for six years, and is Labour's health spokeswoman.
• Deputy leader for the second time. Straight out of Government in 2008, she became deputy to her old caucus friend Phil Goff.
• A popular figure in Parliament, "Aunty Annette" helped MPs accept Andrew Little's leadership when most of the caucus had voted in 2014 for Grant Robertson.
• Has Sri Lankan heritage - her great-great-grandfather was the first Sinhalese settler to arrive in New Zealand.