The new National leader says he expects to be Prime Minister after New Zealand's September election.
Muller has addressed media for the first time after he won the vote for the party's leadership over incumbent Simon Bridges with his new deputy, Auckland MP, Nikki Kaye.
He said New Zealanders had made sacrifices to fight Covid-19 and the economic crisis we face is the biggest in it's history. He conceded the Government's handling of Covid-19 was "impressive".
"But now it's time to take a step forward to rebuild the country, he said.
He said only National can do that.
"The task for the next Government is immense."
His focus will be New Zealand's economic recovery. The size of the task is enormous - "but I will bring my all to it".
He said he would be bringing his skills as a businessman to his role as Leader of the Opposition.
He said National MPs are grounded in the economy. For too long, he said New Zealanders have been invisible to this Government.
Under his leadership, he said it's about what's best for "you and your family".
He said he was not interested in "opposition for opposition sake."
On policies, Muller said they are all still on the table.
That includes tax cuts and an economic response with families at the centre.
Asked if there was room for Bridges on National's front bench, Muller said that was up to Bridges.
He said he wanted to work with the Government on issues, such as climate change. The Climate Change Bill made sense, he said.
On abortion, he said his perspective was influenced by his Catholic faith. It was something "intrinsic" to who he is.
Asked about a reshuffle, he said he will keep Paul Goldsmith in finance. Muller put his arm around Goldsmith while he was speaking.
Asked about any resignations, he said he has not been told that any MPs will stepdown.
He said the decision from caucus to rule out New Zealand First as a support party remains. Asked about working with Winston Peters, Muller said that has not been in front of the caucus.
There is a chance that caucus' decision could be changed.
Muller said he will still hold the Government to account. The Government has talked a big game, but it had failed to deliver. National will meet the challenges this country faces, he said.
He said the first few hours have been "pretty full on".
He referenced the fact that Jacinda Ardern became Opposition leader just six weeks out from the election.
He said he will now focus on New Zealanders getting to know him.
Muller said he was "really hopeful he would be on the news tonight" when asked how he was going to get more well known to New Zealanders.
He wouldn't say who voted for him or go into more details about what happened in caucus - he said the vote was "very respectful".
"We always have moments, every party does."
But he said National remains unified.
"The feeling in that caucus room was incredibly positive."
The message he wanted to portray was that National is a caucus of "remarkable talent".
"This team will pull together a set of policies that New Zealanders will relate to," he said.
The team will spend some time reflecting.
Kaye said Todd Muller was "the most decent person I know".
She said he has "huge integrity".
She had confidence that Muller can unit both the more conservative, and liberal sides of the party.
Muller said Ardern was "impressive" and down to earth.
But he said there were only two or three lifters in the Government's cabinet.
He said New Zealanders will have to look to see which party has the best plan for New Zealand.
He said he has never had a 48 hours like the last two days and that he was "privileged" to be leading National.
Asked about the MPs on stage with him, he said they were just a few "talented MPs" the party has.
They reflect the "strength of the party".
He said he "loves the outdoors" and a "love of single malt whisky".
Bridges gone by lunchtime
Caucus has gone with the unknown candidate over incumbent Bridges and Auckland MP Nikki Kaye has been elected his deputy, replacing Paula Bennett.
Bridges tweeted a photo of his wife Natalie and daughter Jemima, saying: "More time for the most important job I have. Thank you New Zealand."
Meanwhile Muller, who has stayed quiet publicly throughout his short leadership bid this week, released his first statement.
"There is no Team Todd, there is no Team Nikki, or anyone else – there is only Team National," he said.
"National has always been a coalition of city and country, business and community, conservatives and liberals – National is the party for all New Zealanders.
"New Zealanders need a National Government with the experience and management skills to get our country through the worst crisis since the end of the Second World War.
"My focus as leader is our country's economic recovery and the strengthening of every community throughout New Zealand."
Muller is due to make his first public statement at a press conference at 2.15pm. The Bay of Plenty MP has wide name recognition in the agri-business community, but until this week has been far from a household name.
On Wednesday afternoon, after news broke he would be challenging Bridges for the leadership of the party.
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The emergency caucus meeting to resolve Muller's challenge of Bridges comes after two polls put National at about 30 per cent support. The most recent was the 29 per cent in the 1 News Colmar Brunton poll last night.
Muller is the 13th leader of the National Party and is the man who – bar another untimely leadership spill – will face off against Prime Minister Jacinda Ardern in the 2020 election in 120 days' time.
MPs were been behind closed doors of the caucus room for about an hour today where both Bridges and Muller addressed them.
National Party MPs have started tweeting their responses to the leadership change.
Northland MP Matt King said: "Congratulations to Todd Muller on being elected Leader of the National Party. He enjoys the support of the caucus and I look forward to seeing him lead our party to victory and become the Prime Minister of New Zealand."
Christopher Bishop joked: "Last week was Mullet-time. Now it's Muller time!"
Dan Bidois, MP for Northcote, said: "This election is one of the most important we will have for the future of our country. Todd Muller has the economic know how to lead New Zealand through this crisis.
"I support his leadership and look forward to campaigning with him over the next few months #nzpol"
MP for Hamilton West, Tim Macindoe, said: "Today our Caucus was invited to choose between 2 fine NZers. I am grateful to Simon for his leadership of our party, through some incredibly challenging times, & feel for him & his family. Todd Muller will be a great Leader of our party & I'm confident he'll be an outstanding PM."
Ahead of the meeting, both the Muller and the Bridges teams said they were confident of success after three days of intensive phone calls and meetings to try to shore up the numbers.
Muller arrived at Parliament and started his day with a show of chivalry, walking around to open his wife Michelle's door before speaking briefly to the phalanx of cameras.
"It's feeling very exciting, it's a momentous day for the National Party and I look forward to the conversation with my caucus."
Bridges arrived without passing the media.
Other National MPs ran the gauntlet of the media scrums this morning – although very few would say which way they were voting.
Many MPs defended the polling results, saying it was little surprise the party had taken a hit given the extensive media and public attention on Prime Minister Jacinda Ardern during the Covid-19 crisis.
Judith Collins said it was important to have a "very clean vote today and there is not split voting - I'm just very keen to make sure that three months out from an election that the National Party gets ourselves all together, focused, unified and onto the job".
Veteran Anne Tolley said the polls showed Ardern was being rewarded for doing a good job of handling the crisis.
Asked if National could make a comeback from 29 per cent in the polls, she said "of course they can".
"You've had Jacinda on television and radio dominating it for six or seven weeks. It's natural people would react to that. And she's done a good job of keeping the country together, so she's rewarded for that."
Many MPs were also frustrated about the distraction the leadership issue was causing, and concern about the process that had been used.
Who is Todd Muller?
Muller worked in the office of then-Prime Minister Jim Bolger in the early to mid-1990s. Muller was his executive assistant during his second term, when Bolger was rolled by Jenny Shipley.
After he left Parliament, Muller moved into the private sector, working for Zespri in the early 2000s, before moving to kiwifruit and avocado company Apata in 2006, where he was chief executive.
In 2011, Muller move to Auckland to work at Fonterra, where he worked his way up to group director of co-operative affairs.
After Muller left to become an MP in 2014, the co-operative affairs role was taken over by Miles Hurrell – the co-op's now chief executive.
Muller was elected as the Bay of Plenty MP by a country mile; his 21,735 votes put him light years ahead of his Labour Party rival, who won just over 6600.
He took the seat over from former Health Minister and National MP Tony Ryall.
As a backbencher, he served on a number of committees, including Māori Affairs, Social Services, and Education and Science.
He was promoted to deputy chair of a number of those committees and held the coveted position of chairman of the Foreign Affairs, Defence and Trade select committee.
Muller is married to Michelle and has three children.