Mr Muller performed a swift and successful coup this week, ousting unloved leader Simon Bridges off the back of two horror polls for the opposition.
The 51-year-old didn't conduct a single leadership interview in the run-up to Friday's emergency caucus meeting, saying only he had lost faith in Mr Bridges' ability to win government on September 19.
Mr Muller said he decided to mount a leadership bid on Wednesday, saying COVID-19 had provided grounds for a re-think.
"You can't have an event like we've experienced as a country and not have the capacity to re-look at what your priorities are, whether that needs to change and whether there's anything that you need to re-frame up," he said.
With the leadership change, the central premise of the election is now set.
Ms Ardern's Labour will campaign on her government's record, most notably her leadership through COVID-19 which saw her reach record-high approval ratings.
Mr Muller's National will argue their record in previous governments means they are better equipped to handle the coronavirus-induced recession.
"Our number one priority is the recovery of this country, the defence and rebuilding of jobs and communities," he said.
"We've just been hit by the most extraordinary event.
"I'm the first to admit and acknowledge that the government's handling of COVID-19 was overall, impressive.
"To judge a government on the ability to manage a crisis in eight weeks in a health context against ... designing an economy recovery ... I think it doesn't work and I think the country knows it doesn't work."
Mr Muller is a practising Catholic, is married to wife Michelle and has three children.
Known as a pragmatist and a consensus-builder, his chief achievement in parliament to date has been negotiating with the government over the passage of the Zero Carbon Bill, which National supported.
Nikki Kaye, from the party's liberal wing, was elected as deputy, replacing Paula Bennett.
Mr Muller will spend the next few days considering a shadow cabinet reshuffle, any policy switches and the potential for a post-election alliance with Winston Peters' populist party NZ First.
In making the switch, National are making a similar bet to Labour during their 2017 campaign.
Then, Ms Ardern replaced her hapless predecessor Andrew Little just seven weeks before the poll, to great effect.
She produced a double-digit poll bump, providing a basis to form a coaltion government.
After the vote, Mr Muller moved quickly to present a united front.
"There is no Team Todd, there is no Team Nikki, or anyone else - there is only Team National," he said.
Ousted leadership duo Mr Bridges and Ms Bennett are considering their future.