Secret ballots and intense lobbying: How National will select their next leader

National's 56 MPs will vote in a secret ballot to elect a new leader - and the winner has to have the backing of at least 29.

The way it works is that the lowest-polling candidate in the first round drops out and the process goes on until there are two left.

Right now there are three contenders - Amy Adams, Judith Collins and Simon Bridges. There could be a fourth if Mark Mitchell decides to stand, and a fifth if Steven Joyce enters the contest.

It's unlikely any of the three declared contenders will gain an outright majority on the first round, and secondary votes are going to be crucial.

If Collins doesn't have the numbers to survive the first round, her backers would then switch their votes to Adams or Bridges. If Mitchell is in the race, he stands to benefit from that because Collins' supporters could swing in behind him to keep out Adams and Bridges, candidates they didn't favour in the first round.

It's possible the caucus could "do a Jacinda" and end up electing the low-profile MP, hoping he can pull off something similar to Ardern's stunning success.

But it's not so simple for National. Labour was in the red zone when Ardern took over and she pulled it up from an abysmal 24 per cent in the polls, the party's voters came home after switching to the Greens and NZ First.

National is running at over 44 per cent, and according to the latest poll has actually increased its rating since the election, albeit by a tiny amount. Its voters never left home. They're showing intense loyalty to Bill English, and it's going to be a struggle for the new leader to maintain ratings in the mid-40s.

There's another factor which makes picking a winner a fool's errand - the presence of 10 MPs who have only been in parliament for a few months. The rookies won't have had much face time with any of the candidates, although that's going to change in the next two weeks as the lobbying ramps up.

Unlike Labour and the Greens, National Party members don't play a direct role in choosing a new leader. But they can influence it by letting their electorate MP know who they want, and Collins says she knows that's happening because her backers are copying her in on their emails.

She's believed to have much stronger support among party members than she does within the caucus, so she'll be relying on that to put pressure on MPs.

Deciding on the deputy

The deputy's position also has to be decided, which is another complicating factor. Current deputy Paula Bennett wants to keep it, but defeated leadership candidates can and often do throw their names in and the caucus weighs up what it wants the leader/deputy ticket to look like.

It's possible, for example, that if Bridges is defeated by Adams on the deciding leadership vote he could end up as her deputy. It's a ticket that could be attractive, with Adams using her compassionate appeal to take on Ardern while Bridges, aggressive and determined, is her hit man.

Whichever way it works out, the main players will also have their longer-term futures in mind. This won't be the last leadership contest before National again takes power. It's highly unlikely to oust Labour after one term, and the losing leader won't survive another three years.

- NZN