As National MPs prepare for tonight's critical crisis meeting in Parliament, Judith Collins and Simon Bridges are the front runners to take the reins of the embattled National Party.

As the last of the National MPs filter back into Wellington, there may well be quite a battle for the deputy leadership position tonight as well.

Rodney MP Mark Mitchell is understood to be considering a tilt, as is Hutt South MP Chris Bishop.

Long-time MP Gerry Brownlee might also be an option for the deputy role, as is finance spokesman – and Bridges backer – Paul Goldsmith.

Current deputy, Nikki Kaye, is likely to be chairing tonight's meeting.

She would not be drawn on whether she would put herself forward for leader, or deputy leader, or who she'd support.

There is no script as to how tonight's meeting – understood to be starting around 7pm – will run, but there is every chance a new leader will emerge tonight.

Speaking to reporters on the way into Parliament this morning, Mitchell said tonight's caucus meeting was about deciding on a process to follow.

"There may be an outcome tonight, there may be one tomorrow – we will have to wait and see."

This morning, Muller announced his he was stepping down as National leader – a position he has held for less than 60 days.

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• The Huddle: What went wrong for Todd Muller?
• Audrey Young: Mental health behind Todd Muller's resignation
• Judith Collins emerges as favourite to become National leader

"The role has taken a heavy toll on me personally, and on my family, and this has become untenable from a health perspective."
The shock resignation led to a wave of speculation as to who would be taking over.

The clear front runners are Bridges – who Muller rolled as leader in late May – or Collins.

Both have run for the leadership before – Collins twice.

The pair squared off after Bill English stood down in early 2018; Bridges won that contest.
But neither MP was giving much away this morning.

Bridges ignored questions as to whether he was running for the leadership.

Speaking to reporters at Auckland Airport, Collins said "I'm going to keep all of my discussions within caucus" when asked the same thing.

Asked if she had been waiting a long time for her shot as leader, Collins said today was "just another day".

National MPs learned of Muller's resignation on a call this morning at 7.30am. Moments later, a media statement was released.

The party's senior whip, Barbara Kuriger, said she did not know whether Muller would be in Wellington for tonight's meeting.

National MPs have been making their way back to the capital all day.

Almost all MPs door-stopped by media at an airport, or in front of Parliament, today sent Muller and his family their sympathy and best wishes.

"The important thing is we have compassion for Todd at this time while we work through what has been a very difficult time," Kaye said this afternoon.

Speaking to reporters in Wellington airport, Paula Bennett – who announced earlier this month that she would not be contesting the election – would only say the caucus has a "big decision to make".

She said she had "no comment to make about anything", when repeatedly questioned on today's developments and what will happen tonight.

But she did say: "These are just difficult times for the party and the caucus – so we are just going to work our way through that respectfully."