Ardern choked back tears on Thursday as she said she did not have the energy to seek re-election.

“I know what this job takes. And I know that I no longer have enough in the tank to do it justice,” she said.

“I would be doing a disservice to continue.”

Ardern said the Labour party would vote on a new leader on Sunday but her deputy Grant Robertson would not be a candidate.

The bombshell announcement came in Napier at the party’s year-starting caucus retreat.

Ardern was expected to announce an election date – which she did, for October 14 – but then stunned all-comers by declaring her exit from politics.

She said she left without regrets and offered a simple way she would like to be remembered.

“As someone who always tried to be kind,” she said.

Ardern also addressed her family – her fiance Clarke Gayford, who sat in on her press conference, and her four-year-old daughter Neve.

“Neve, mum is looking forward to being there when you start school this year,” she said.

“And to Clarke – let’s finally get married.”

Jacinda Ardern with partner Clarke Gayford after announcing her resignation. Photo: AAP/Ben McKay

The race to succeed Ardern as prime minister appears to be an open contest with Robertson – the finance minister and a previous leadership aspirant – deciding not to stand.

“It has been the honour of my working life to have supported Jacinda as Minister of Finance and as Deputy Prime Minister,” he said in a statement.

“Her intellect, judgement and empathy mark her out as one of New Zealand’s finest leaders.”

“As a colleague, a friend and a New Zealander I am incredibly grateful for her service and commitment and wish her every joy and success in the future.”

Others who could stand include education minister Chris Hipkins, a close ally of Ardern, foreign minister Nanaia Mahuta or immigration minister Michael Wood.

Under Labour rules, two-thirds of the caucus must support a candidate for the leadership at Sunday’s vote or the contest will be thrown open to the wider membership.

Ardern – who will continue as MP for the Auckland electorate of Mt Albert through to April – has requested this process be complete by February 7, the day after Waitangi Day.

Australian Prime Minister Anthony Albanese said Ardern had “shown the world how to lead with intellect and strength”.

“She has demonstrated that empathy and insight are powerful leadership qualities,” he wrote on Twitter.

“Jacinda has been a fierce advocate for New Zealand, an inspiration to so many and a great friend to me.

“I wish (her) and her family well in the next chapter of their lives.”

Albanese said he looked forward to working with the new leader.

– AAP