Former National party leader Don Brash is throwing his support behind Judith Collins for leader of the Opposition.

In a bolt out of the blue, leader Todd Muller resigned this morning.

Brash told Heather du Plessis-Allan Collins is experienced in Government and Opposition - unlike any other contender.

"She's very experienced at eight or nine different portfolios - Justice, Police, Corrections, you name it - and she's fast, she's quick on her feet, she handles the media well."

He says that she also has experience in opposition as well as Government, which will be a key skill as the Leader of the Opposition is a tough position. 

"The Speaker of the House told me that once, that you're expected to have an informed opinion of everything the Prime Minister has an opinion on, and you've only got a tiny fraction of the Prime Minister's resources."

Brash says that the 'Dirty Politics' saga shouldn't affect her, as her new memoir details how that case was overplayed. 

Collins isn't giving away her position on the leadership - telling media she's keeping all her discussions for the Caucus.

Brash says that if it isn't Collins, then Bridges is the second choice. 

"The public may not have liked Simon, but they respected him."

He does not believe that Nikki Kaye will put her hand up.