Former National Leader Todd Muller will leave politics at the election in October.
The announcement has triggered a minor reshuffle of the party's portfolios.
Muller told RNZ he had "an interesting ride over nine years" but weighed up his options and decided now was the time to leave.
"Really it's just not enough fire in the belly for the job that you need to do, particularly as an MP and particularly if we win in October.
"You really need to be 120 percent for this, and be able to give a huge amount of hours - and I just don't have the enthusiasm to do it."
He said Luxon and deputy leader Nicola Willis had "completely reset the caucus" and he believed they would win the election.
"That's the harder part of the decision frankly ... that was the 'reasons to stay' on the side of the list, on the other side you've just got to be really true to yourself in terms of where you're at, the needs of your family, do you have the capacity to give it the hours and dedication needed.
"It'll be a scrap in the next few months without a doubt. This is a change election not a 'don't put it at risk' election."
He said he was feeling relieved by his decision but had not yet reflected on what he would do next.
"No, I'm smiling too broadly for that. There will be a time when I can sit and reflect but no, there's time for that."
His family was supportive of his decision too, he said.
"Exceptionally happy, they are very pleased they're going to see a bit more of me - and a bit less of me on the side of my car which my youngest thinks is a great outcome."
In a statement on Thursday morning, he said he had wanted to be an MP since he was a young boy.
"I am a changed person through my parliamentary experience, and I hope adversity has broadened my understanding and care for the many forms of exasperation and inspiration that exist in our community."
In a statement, Luxon said Muller had been a deep-thinking, hard-working member of National's team.
"Todd gives 110 percent of himself to whatever he does. I wish him and his family all the very best for the future," he said.
"I thank him for his service to the party, the public and the people of the Bay of Plenty."
Muller has been the MP for Bay of Plenty since the 2014 election, and most recently held the Agriculture and Climate Change portfolios.
Luxon said Todd McClay would take over Agriculture - handing McClay's Tourism role to Joseph Mooney - and Simon Watts would pick up Climate Change.
Luxon also announced Chris Penk would take over Cyclone Recovery.