It could be all over by Monday – or the process could drag out for months, writes constitutional law expert Andrew Geddis.

Oh no! We’ve gone and broken our prime minister, that was careless of us!

It is all somewhat “shocking but unsurprising”, to quote Aotearoa New Zealand’s finest summary of news and the zeitgeist. Looking back over five-and-a-bit years of having to deal with Winston Peters, terrorist mass-murder, a global pandemic, volcanic disaster, and now the malfunctioning of the international economy makes you amazed Jacinda Ardern lasted as long as she did in the job. That said, her resignation this early in the election year certainly wasn’t front-and-centre on many people’s political bingo cards for 2023.

Yes, in hindsight we really should have treated her a bit better. But now I guess we have to go out and get ourselves a new PM?

Yep. Can’t be without a PM, I’m afraid. Interestingly (for those of a certain nerdy disposition), there’s no piece of legislation that establishes the role. Rather, our entire constitutional set-up is predicated on there being someone filling that office to act as the head of government. The Cabinet Manual, which functions as the government’s overall “How To Government” guide, explains why that is in more detail here.

OK, so from where can we get ourselves a replacement PM?

There’s only one place for the governor general to source a new PM from: the House of Representatives. The PM must first be an elected member of parliament.

Wait up – what’s the governor general doing in this picture?

In formal legal terms, the PM is appointed (given a warrant) by the governor general.

So, it’s Dame Cindy Kiro who gets to choose our next PM from out of all the MPs? Toni Severen is in with a chance, if she sends along a nice enough gift basket?

No, don’t be silly. We may still cling to some ridiculously outmoded symbolic concepts of public authority (*cough* King Charlie, long to rule over us *cough*), but not to that degree. The governor general doesn’t individually choose the PM, but rather will only recognise as PM the individual MP who “enjoys the confidence of the House”. In other words, whichever MP has the support of a majority of other MPs as being PM will automatically be appointed to that role by the governor general.

Does that then mean we have to wait until parliament gets back together to have a new PM in place?

Nope. Because the governor general knows how our political system works, and the governor general can count. At the moment, the Labour Party has an absolute majority of the seats in the House of Representatives. And, the MPs from the Labour Party will (barring some catastrophic political meltdown) support their party leader as being PM. Meaning that the governor general will appoint as a replacement PM whomever the Labour Party MPs recognise as being their new leader.

So, it’s the Labour Party who will really choose our new PM? How do they go about doing that?

According to their own internal party rules. Basically, these say that where the party leader quits, the Labour Party MPs have seven days to try and choose a new leader from among themselves. Doing so requires at least a 66% majority in caucus (or, 43 of the party’s 64 MPs). If the MPs can’t do so in that timeframe, then a wider election process is triggered involving individual party members, affiliated unions, and MPs each getting a say on who they think the leader should be.

That sounds like it might be a bit complicated – how long are we looking at here?

Ardern has stated that February 7 will be her absolutely last day in the role of PM, which I guess is designed to put a fire under the replacement process (as well as allow her to be at her daughter Neve’s first day of school, as she promised in her resignation announcement). But a replacement could very well be in place sooner than that. The Labour caucus is meeting on Sunday to see if they can get the requisite two-thirds agreement. If they can, we will have a new PM by next week.

However, if the MPs can’t agree and the issue has to go to a wider, party-wide selection process, a replacement leader realistically won’t be chosen for at least a month or two. Assuming Ardern holds to her resignation deadline, the Labour MPs will then have to agree on some interim figure to step up as PM until the party chooses its permanent leader. That’s not a major drama – remember that Labour has an absolute majority in the House, so whomever its MPs agree should act as PM will be appointed as such by the governor general.

So, that’s how the process will work. Who will be the country’s new PM at the end of it all?

Rachel Brooking. New Zealand’s next PM will be Rachel Brooking. You read it here first. Remember that. Rachel Brooking is going to be our new PM.

Follow Gone By Lunchtime on Apple Podcasts, Spotify or your favourite podcast provider.