She had appeared to forget a fateful day on the election trail, in which she was asked if she would resign if she introduced a wealth tax.
She had replied by saying it was hypothetical because it would not happen and then "I won't allow it to happen as Prime Minister."
Later in the day, all of this resulted in something of an Alice down the rabbit hole scenario at Ardern's press conference.
Ardern insisted nothing had changed in her positions. Labour's tax policy was the same as it ever was, and no work was being done on it. There was nothing to announce now - Labour was not doing anything on taxes. It escalated to the point she would not even say whether Labour intended to review its tax policy ahead of the 2023 election.
What we know is that Ardern has both ruled out a wealth tax until the end of time and not ruled out a wealth tax after 2023. What we do not know is whether Labour will have a tax policy at all in 2023.
She is not the first PM to say something will not happen on her watch – Sir John Key did the same on the retirement age.
It is a card that is used for short-term political expedience - usually to try to kill off a rival's attacks or suspicions of a secret agenda during an election campaign.
But they come with a longer-term cost and that is why they are rare. They are also cowardly.
Prime Ministers can backtrack on some promises if there is a compelling reason to do so.
After saying it would not increase GST, National did so and sold it as the only way for it to be able to afford to deliver on its bigger campaign promise: income tax cuts. It got away with it because of the global financial crisis: and the tax cuts.
But once you've said you would resign if you ever introduced said policy it becomes almost impossible to back away from.
They back Prime Ministers into corners they cannot get out of, even years later if the circumstances have changed drastically and the case for the policy they have rejected becomes compelling.
Ardern has now done it at least three times. She moved early to swear she would not raise the retirement age in 2017 - prior to that, it had been Labour policy to raise it and it had tried to accuse National of being irresponsible for not considering it. Then came her decision on the capital gains tax, and now (maybe?) the wealth tax.
Of the three, the capital gains tax was the most significant: she scotched it to neutralise a policy that was potentially popular rather than try to sell the need for it.
If she does not sometimes regret that call, she should.
Such pledges are also not true to your own party – and leave supporters disappointed. If Ardern does not regret making the call she did on the capital gains tax, she should.
The tax debate can get to the slightly ludicrous stage in which there is a shock and horror reaction to National's leader Christopher Luxon saying he wants to remove the top tax bracket – something that was in Labour's tax policy – but always opposed by National.
Meanwhile, Ardern is being criticised for the possibility she might (or not, who knows?) be entertaining a wealth tax.
Again, making the wealthy pay more is Labour philosophy (but not yet policy possibly) and not National policy.
In short, they are coming under fire for not holding their opponents' policies.
That is the whole point of democracy.
