After 30 years, National MP Nick Smith is leaving Parliament after details of an inquiry into a verbal altercation at his Wellington office was leaked to the media.

Smith, one of Parliament's longest-serving MPs, has announced he will retire from Parliament on June 10.

He says Parliamentary Services has been conducting a confidential inquiry into a verbal altercation at his Wellington office last July, which has not yet concluded.

"I was advised on Friday that the inquiry and its details have been leaked to the media for release tomorrow. It is inappropriate for employment disputes to be litigated in public," he says.

He says he regrets the incident, for which he has apologised - both on the day and again now.

"I have decided the best course of action for the parties involved, the National Party, my family and myself, is to retire now."

He says politics places a significant burden on family.

"I am incredibly grateful for the support of my wife, children and wider family. There have been recent changes in family circumstances which require me to give greater support."

He says it's an opportunity for National to renew itself, and the leader, party and caucus has his full support.

Smith has served as minister of 15 different portfolios and represented the Nelson region for 30 years until he lost his seat at the 2020 election.

"It was working for constituents and advocating for the region that I enjoyed most and I have come to realise that the role as a List MP is just not me."

He had earlier this year decided to retire, he says, and the only question was when.

He will re-join his family's crane and construction business.

National Party leader Judith Collins thanks Dr Smith for his service and dedication to Nelson and the wider New Zealand public over the last three decades.

"He's a hardworking and extremely competent Minister in the Fourth and Fifth National governments.

"His relentless advocacy for the Nelson region has been tireless, including recently as he stood up against the amalgamation of NMIT and the closure of Nikau House," she says.

She says he created 17 marine reserves during his period as Minister and was the lead advocate for the Kahurangi National Park.

She will not comment on the Parliamentary Service inquiry.

"As there is an ongoing investigation into an employment dispute, it would be inappropriate to comment further on that matter."

-RNZ.