New Zealand Prime Minister Jacinda Ardern has announced her retirement from politics within the next month.

Ms 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.”

Ms Ardern said the Labour party would elect a new leader by the weekend but deputy prime minister Grant Robertson would not be a candidate.

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

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

She said she left without regrets, offering a simple way she would like to be remembered.

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

Ms Ardern also addressed her family; her fiance Clarke Gayford and 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.”

By Ben McKay in Napier

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