Former National Party leader Todd Muller has announced he will not stand again at the next election.

Muller said it was a "hard decision".

National Party leader Christopher Luxon said Muller was a "deep-thinking and hard-working" member of the caucus and he respected Muller's decision to "call time".

The Bay of Plenty MP was National Party leader from May to July 2020, rolling Simon Bridges. He stood down citing mental health challenges and was replaced by Judith Collins.

In a statement, Muller said he had wanted to be an MP since he was a “young boy”.

“When I entered parliament in 2014, I was determined to bring the best of myself to this special place. In the subsequent nine years I have tried to give my absolute all for my community and my country.”

He said he had lost “enthusiasm and energy” for the job, a conclusion he’d come to after much reflection and discussion with loved ones.

“I will not contest the next election. I cannot recommit to giving the National Party nor my Bay of Plenty community another full three-year term.”

He said the upcoming general election was the right time to stand down and allow a new candidate with “fire in their belly” to take on the role.

“I can't deny this is a hard decision.”

He said Luxon, who succeeded Collins, as well as his deputy Nicola Willis had “completely reset” the National Party caucus.

“I firmly believe they will provide tremendous leadership at a time of great challenges for our country come October.

“Their support for me personally and professionally has been unequivocal and I wish to thank them for their genuine values-based leadership.

“My journey in politics has been a stimulating, challenging and at times difficult ride. However, I am thankful for it all.”

He said he was a “changed person” from my parliamentary experience, and hoped adversity had broadened his understanding “for the many forms of exasperation and inspiration that exist in our community”.

Muller said it had been an “absolute privilege” to have been elected three times by the Bay of Plenty electorate.

“To my family, thank you for your patience and support as I’ve served our community, often at the expense of time with you. You are my rocks and I love you very much.”

Muller has previously worked for Zespri and Fonterra, and was first elected in 2014, under the leadership of former Prime Minister John Key.

He led the National Party in a torrid period that included scandals about a Make America Great Again hat he had on display in his office and when his deputy leader Nikki Kaye incorrectly claimed Paul Goldsmith was Māori.

He received praise for his candour and honesty about his struggles with mental health and anxiety while National Party leader.

It is also not the first time he has announced his retirement.

Muller attended Tauranga Boys' College and has a Master's degree from Waikato University.

Rotorua MP Todd McClay will replace Muller as agriculture spokesman, while North Shore MP Simon Watts will take on his climate change role.

McClay would be relieved of the Tourism portfolio by Southland MP Joseph Mooney.

Luxon said Muller gave "110% of himself to whatever he does" and wished him and his family all the best.