Jacinda Ardern's stunning leadership story is the stuff of legend.

And the legend just grew as the leader of a 7.2 per cent party, New Zealand First, crowned her Prime Minister 80 days after she became Labour leader.

A Labour-led Government has been a possibility almost all of the past three years, most polls have shown.

But the reality still comes as a shock, not least to Labour itself. As various MPs, staffers and officials crowded into the caucus room for her acceptance speech tonight, it was a pinch-me moment for them too.

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Ardern will be something of a stranger as Prime Minister. She has barely been heard from in the past month during a self-imposed exile.

She was Labour leader for just two months of high-pressure scrutiny before the voters cast their judgment.

The extreme pressure began this evening as Winston Peters announced his decision to her through television.

But as in almost all of her public outings, she acquitted herself very well.

Her first challenge will be to weather the perception of many that she did not earn the job, or that she does not deserve the job.

That will be an inevitable consequence of the process in which she was "gifted" the role by the father of the House.

That perception will disappear with the passage of time. Bill English's gracious concession speech suggests that that is a line National is not going to pursue, lest they look like sore losers.

Anyway, the thing that will come to matter most for Ardern is not how she got there but what she does.

For New Zealand First there were major risks with either decision, National or Labour.

Peters can hardly be blamed for having exercised the power that the election delivered him to drive a hard bargain.

Going with National risked being associated with the stagnation that would almost certainly have afflicted a fourth-term Government with whom he had history.

A deal with a new leader, and in what she said would be a real partnership, must have been attractive.

There was no great mood for change before Ardern became leader. And the election result could hardly be called a major endorsement for change. But more wanted change than not and the numbers don't lie.

New Zealand is entering a new era in which its most experienced and wily politician has handed power to the least experienced leader who has become the country's second-youngest Prime Minister.

It is going to be one fascinating ride.