Ms Ardern stunned all-comers on Thursday when she announced her imminent departure from the role, citing exhaustion.
"I know what this job takes and I know that I no longer have enough in the tank to do it justice. It's that simple," she said.
The Labour leader's decision has prompted an outpouring of tributes, from prime ministers to prominent actors, her most fervent fans to begrudging critics.
Ms Ardern came to represent a new style of compassionate, quietly determined leadership.
For those on the left, she was a political rock star.
And nowhere will her absence be felt more keenly than in the New Zealand Labour party, which has nine months to pick up the pieces before a national election.
From 2008, Labour spent nine years in the political wilderness before Ms Ardern took the leadership and led the party to office in 2017, courtesy of an out-of-the-box election campaign and canny coalition-building.
In 2020, she won re-election with the biggest single-party vote since World War II, a result of her early response to the COVID-19 pandemic.
Even as her popularity wanes - just as many incumbents leaders did during the pandemic - her support in the Labour caucus remained resolute.
Labour MPs were stunned by Ms Ardern's departure; several struggling to string together words as they spoke with journalists on their way from the party retreat in Napier where they had assembled for some January team-building.
Their next act will be to regroup and pick a new leader, with barely a day to waste.
As her last act as prime minister, Ms Ardern named the election date, picking October 14 for Kiwis to go to the polls.
She also set in train the process to replace her, which begins with a caucus meeting in Wellington on Sunday.
As per party rules, two-thirds of the 64-strong caucus - or 43 MPs - must support a new leader at that meeting or the decision will be thrown to the wider party membership.
On Thursday, no Labour MP would commit to running for the job, though a few ruled themselves out - most notably Grant Robertson, Ms Ardern's deputy and a former leadership candidate.
But many think the 51-year-old would be open to reconsidering should the party beg him to.Â
Other possible contenders for the top job include Education Minister Chris Hipkins, Health Minister Andrew Little, Justice Minister Kiritapu Allan and Immigration Minister Michael Wood.
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