Everyone knew Ms Ardern was under personal and political pressure. She has had an exhausting, crisis-filled five years as prime minister, and she was visibly strained at the end of last year after a turbulent final few months of the parliamentary year.
But no-one expected that she was on the verge of quitting, especially with an election looming.
There is precedent for this. Former National leader John Key stepped down at the end of 2016 to make way for Bill English to place his feet under the prime ministerial desk, and whoever the Labour caucus backs this weekend will similarly have time to show their wares to the electorate before voting starts for the October 14 election.
For Labour’s caucus must elect a new leader this weekend: a contested race which required the wider party vote would send a terrible signal of disunity and perhaps mean whoever took over as prime minister would have no chance of winning the election.
On that note, it is surprising that South Dunedin’s favourite son, Grant Robertson, immediately ruled himself out of the running.
Had he wanted the leadership there is little doubt that Labour would have given it to him, and that he would almost certainly have done an excellent job.
Labour’s twin towers stepping aside means a baptism of fire awaits for whichever senior minister feels up to the task of leading both the party and New Zealand.
They will not have expected to have found themselves at the top of the greasy pole, but then neither did Ms Ardern.
She was catapulted to Labour’s leadership just weeks before the 2017 election after a similarly surprising resignation by then leader Andrew Little and, as she said yesterday, she was not expected to become prime minister.
Although there will inevitably be naysayers, the majority of New Zealanders would agree that once she did assume power, she demonstrated a compassionate brand of leadership which resonated not only throughout this country but also overseas.
Her empathy, warmth and caring — as well as a hint of steely resolve — was exactly the right approach in the aftermath of the appalling terror attack on the Christchurch mosques in 2019, and will be an abiding memory of her leadership.
The other defining issue of Ms Ardern’s prime ministership has been Covid-19.
The hateful and appalling attacks she has endured from a rabid minority for putting in place the public health measures her advisors recommended she adopt to try to prevent the spread of the pandemic disease in New Zealand should not overshadow the fact that, for the most part, they were effective.
New Zealand enjoyed a year or more with Covid-19 largely being something that happened overseas. In many of those countries thousands of lives were lost and the health system was overwhelmed by desperately ill people.
This country however, brought in draconian restrictions which grated for many, but which most would now recognise bought the Government time to wait for a vaccine to be developed and then rolled out across New Zealand. The crammed, desperate emergency departments experienced in other countries at the start of the pandemic did not unfold here, and much of the credit for that rests with Ms Ardern.
Her final term as prime minister has been scarred with division over water assets and the concept of co-governance, controversial issues which have shorn some of the lustre from the Ardern premiership and seen her personal approval rating and preferred party polling both slip.
With her departure Labour loses both its best political asset but also possibly its biggest political liability.
Weighing on Ms Ardern’s thinking must have been the possibility that her leadership could be an anchor which might arrest her party’s bid for a third term.
Ms Ardern’s legacy will be a complicated one, but her best moments transcended party.
For five years she urged New Zealanders to be kind, and few will begrudge her decision to now be kind to herself and her family and to step away from the often harsh world of politics.
