It became clear in the first two weeks that Ardern was going to rescue Labour from oblivion.
As the days have turned to weeks it has now become clear that she is a serious contender for Prime Minister after September 23 - although the decision still looks likely to rest with New Zealand First.
New Zealand, it seems, has been almost itching for its own political quake.
Ever since the popularity of independent left-wing US Senator Bernie Sanders, the Brexit vote, the shift in support which gave Donald Trump the US presidency, and Jeremy Corbyn's humiliation of Theresa May, pundits have been looking for similar seismic shifts in New Zealand politics.
Winston Peters has been compared to Trump by his detractors. Metiria Turei was compared to Sanders by her own campaign manager.
Ardern has been compared to Canada's popular Prime Minister Justin Trudeau, France's young president Emmanuel Macron and even former British Prime Minister Tony Blair, for whom she once worked.
When Labour was elected in 1997 under Blair, it broke a spell of 18 years in the Opposition wilderness for UK Labour. Blair was the last Labour leader to win a British election. His victory in 2005 also marked the last time a New Zealand Labour leader has won - Helen Clark for a third term.
Former British Labour MP and leftist Bryan Gould, who reviewed New Zealand Labour's poor performance last election, has been one of those watching the rise of Ardern with fascination.
Gould said he could see some why some people drew comparisons with Blair.
Both were young, very presentable and were a break with the past.
But the question at the back of a lot of people's minds about Ardern was how serious she was and whether she was up to it.
"I suppose, and probably quite unfairly, it is a question that arises because she is so strong in terms of presentation and voter appeal."
"I must say, I think so far she has not only answered that question but almost made it irrelevant, perhaps slightly to my own surprise.
"She has been pretty sure-footed and has handled things well."
But Gould said Ardern could not be compared with Blair because he built his image by doing things that were deliberately contrary to Labour's traditional policies and Ardern had not done that.
Kiwi David Howman, who lived in Montreal for 13 years as the director general of the World Anti-Doping Agency, met Justin Trudeau before he became leader of the Liberals (he knew his godfather and had met through an interest in sport).
Plenty of people had thought he was too young and experienced when he first led the Liberals but Howman said he ran a campaign which was fresh and appealing to young people.
After Trudeau beat the Prime Minister of 10 years, Stephen Harper, the inauguration in Ottawa was attended by thousands of people celebrating in the streets.
Howman didn't want to comment on Ardern. But on Trudeau, he said; "That he was so successful indicated how compelling he was."
There are 28 days until voters go to the polls. What seemed a foregone conclusion a month ago is now a very different picture.
Prime Minister Bill English said the dynamics of what was happening were pretty clear.
"The stardust is still settling," he said.
"People are now pleased there is a tough competition for the election because National hasn't really had that for the last wee while.
"They now have to think about the comparisons and we welcome that."
English concedes that Labour could overtake National in party vote polling during the campaign.
"We've been working pretty hard for it to lift up going into the election as people make the choice."
Voters were only just getting to the phase where they had to start thinking about the change on their choice at the election.
"It has got them interested, that's for sure. They know there is a competition and they like that," English said.
Such has been Ardern's impact that in last week's TV1 Colmar Brunton poll she drew level with English on 30 per cent as preferred Prime Minister.
The proximity of Labour's leadership change to the election may have concentrated the effect of the change.
If Andrew Little had resigned a year ago, the effects and the interest in it would likely have been more diffuse.
