A chance the National Party may make a decision on a new leader tonight.

Leader Todd Muller took the country by surprise by resigning this morning, and the Caucus is holding an emergency meeting in Wellington tonight.

Former National Party Prime Minister Jim Bolger is surmising they'll want to make a decision tonight to remove the uncertainty.

But he told Heather du Plessis-Allan it's whether they've had sufficient time to do the detailed thinking - rather than just rush to nominate someone who turns out to be a mistake in a weeks' time.

"So I'd be much happier if they took a day or two. That would make no difference, but picking the right person would make a huge difference."

He says we should be talking about policy rather than people.

"The person who's got the imagination and intellect to describe a different model and put the policies in place to achieve it."