When an ashen-faced Andrew Little told 1 News that mid-20s polling had prompted him to talk to colleagues about resigning, it triggered a rapid sequence of events that ended in Ardern being made leader, with Kelvin Davis her deputy.
Most of the crowd who turned out on Sunday would have stayed home but for that - and just the fact of their elevation, but those impressively sure-footed first few days, beginning with that universally acclaimed press conference on Tuesday.
Part of that turnaround - the appearance, at least, of a race turned on its head - is what has happened to National. The most effective campaigning machine in New Zealand politics has had its guns spiked.
Experts at opposing the opposition, National has looked flat-footed, its arsenal uncharacteristically blunted. Labour will literally have to tear down its billboards, but National will be tearing up much of its own campaign strategy.
Steven Joyce, a world-class rapid response unit, was quick to denounce Labour's policy today, saying, "the Labour Party might have changed its leaders but where it wants to take New Zealand hasn't changed".
And that has been the essence of their response to the Ardern factor so far: same old Labour.
But it isn't the same, not as far as the counterattack strategy is concerned. The "Angry Andrew" chapter in the National Party playbook has been shredded.
Joyce was almost unrecognisably reluctant to point to any failures on the part of the Labour leader on RNZ Morning Report on Thursday. Clearly, a decision had already been taken to shelve anything in the key of, say, "Zip it, sweetie" (Paula Bennett to Ardern). Or Nikki Kaye's description of Ardern's promotion to deputy leader as a "superficial, cosmetic facelift", or her claim she had "absolutely failed our generation".
And you can be confident Maggie Barry won't be reheating her barb in Parliament questioning whether Ardern was entitled to speak on paid parental leave given she doesn't have any children. The fallout from some of the questions Ardern faced in her first couple of days would make anything in that area look extra toxic, not that most National MPs wouldn't have any appetite for it anyway.
The question of Ardern's experience and substance is a different matter. But that, too, is fraught. Especially in the proverbial honeymoon phase. The Newshub take on Bennett's appearance on The Nation was headlined, "Jacinda Ardern lacks 'the kind of brain' to unite Kiwis, says Paula Bennett".
The headline was hyperbolic and unfair. It has since been changed to "Jacinda Ardern lacks substance compared to Bill English - Paula Bennett". (What Bennett in fact said in the interview, recorded Friday, was: "I just think what [voters] are also looking for is substance and someone who's got the kind of brain to pull this country together and has got a proven record", prompting Lisa Owen to interject, "Doesn't she have substance?" and Bennett reply: "I just think he's got a bit more is all. I think she does, but I think that he equally has a proven track record ...") But it is an instructive episode. Any shade cast on Ardern that looks imperious or patronising, anything that looks even distantly like bully tactics, will make headlines.
It would be easy to overstate the extent to which National has been hobbled. They're not so much punch-drunk as waiting out a round or two. Labour's press coverage has been extraordinary. The stories about Jacinda's big Saturday of interviews, rugby league, door knocking and MCing a school quiz are the stuff of spin doctors' dreams. But the trip won't last forever: just stand back and wait for the comedown. This is only day five of Ardern's leadership. It is hard to imagine the next five going quite so exceptionally well.
The exit of Little plus the semi-resignation of Green co-leader Metiria Turei will give National cause to cheerfully remark that they find it hard to keep up with just who the leaders are on the left. Labour might be resurgent, but it would take a mighty surge to put them in a position in the polls showing they no longer need both the Greens and NZ First to govern. And, Ardern or not, Joyce and co will be delighted to paint that as a horrifying three-part disharmony.
The next poll or two are the most eagerly anticipated for a long while. Anecdotally, it seems very likely that Ardern's Labour will have won back not just support from the Greens but also from the liberal edge of National. But National have defied predictions of a slump before. Plenty of times.
Most of all, National will bang the economy drum. Economy, economy, economy.
For them a blessing would be a repeat of something like the 2011 "Show me the money" moment, in which John Key mocked Phil Goff over his knowledge of his own party's costings.
But in seeking any such sequel, English won't want to fall into condescension. Unless panic were to take grip, you wouldn't expect English to go full Key and start jelly wrestling on breakfast radio. But the rise of Labour under Ardern could very well embolden National to rummage around in their hat for a policy rabbit.
A final clue from Sunday on National's counterattack strategy came in the form of a late-afternoon campaign tweet:
A reasonable response to this is: What? Is this some comparison to the actor/wrestler? Or just a rock? Why would you do that?
In fact it's an abstruse reference to this Q and A, where Deputy Davis read out a shopping list of insults, among which was the assertion that Prime Minister English has the personality of a rock. You're asking a lot of your Twitter followers to have absorbed all that, but the sought narrative is clear enough: wouldn't you rather a substantial, dependable leader than an unproven, charismatic novice?
It would be overdoing it to infer disarray based on one weird tweet. They'll be hoping, however, that the message is not so clumsily phrased next time.
This article was first published on The Spinoff and is reproduced with permission.
