In a statement Muller said he had taken time over the weekend to reflect on his experience over the last several weeks as leader of the opposition.
"It has become clear to me that I am not the best person to be Leader of the Opposition and Leader of the New Zealand National Party at this critical time for New Zealand," he said.
"It is more important than ever that the New Zealand National Party has a leader who is comfortable in the role.
"The role has taken a heavy toll on me personally, and on my family, and this has become untenable from a health perspective.
"For that reason I will be stepping down as Leader effective immediately."
He said he intended to take some time out of the spotlight to spend with family and restore his energy before reconnecting with his community.
"I look forward to continuing to serve as a loyal member of the National Party team and Member of Parliament for Bay of Plenty."
Muller was elected as a new MP for the Bay of Plenty in 2014. He took over the leadership after rolling Simon Bridges in May.
- 21 May - Bridges summons National MPs to Parliament for an emergency caucus meeting to decide who will lead the party, after two abysmal polls appeared to have sealed his fate.
- 22 May - Todd Muller is the new National Party leader, with Nikki Kaye as new deputy.
- 25 May - Muller reshuffles shadow cabinet, Amy Adams has backtracked ranked third in caucus, Gerry Brownlee takes over National's election campaign as chair.
- 7 July - National Party MP Hamish Walker admits to to leaking Covid-19 patient details. Former National Party president Michelle Boag says she passed on the information.
Prime Minister Jacinda Ardern passed her best wishes to Todd Muller and his family.
Ardern said no matter what side of parliament you're sitting, politics is a difficult place.
Palmerston North lawyer and conservative political commentator Liam Hehir told Morning Report the resignation was "bizarre and unprecedented".
"The only thing that is really clear is what a disaster it is."
Two months before an election the only option for the party was disaster control, he said.
He said it was a difficult time for anyone to take the leadership, but they would not be held responsible for any election result, which could make the job more attractive.
This article is free to read, but it would awesome if we had your support.