The new National leader says he expects to be Prime Minister after New Zealand's September election.
Muller is addressing 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 emense."
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."
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.
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 about a reshuffle, he said he will keep Paul Goldsmith in finance. 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 was one of high promise, and low delivery.
He said the Government has talked a big game, but it had failed to deliver.
He said National will meet the challenges this country faces.