The National Party is unaware if Dr Nick Smith will return to Parliament after announcing his resignation while revealing a complaint against him for a "verbal altercation" with a staffer.

His colleagues have also denied any knowledge of the alleged incident nor the ongoing inquiry.

The veteran MP said on Monday he would leave Parliament on June 10, and revealed Parliamentary Service was investigating "a verbal altercation in my Wellington office last July that has not concluded".

He has since gone to ground, though National Party deputy leader Dr Shane Reti didn't know if Smith was on annual leave or sick leave.

Reti said on Tuesday morning the first he had heard of the investigation into his colleague was through media.

"I was surprised," said Reti, speaking in place of party leader Judith Collins, who was absent while attending her son's graduation.

Reti had spoken to Smith, but was unsure when, or if, he'd return to Parliament.

"All I know is he is not here this week, there will be discussions on the weekend about next week. That is a question for Dr Smith.

"I respect Dr Smith's decision to not be here this week while he figures things out."

Smith said he was told on Friday that the inquiry and its details had been leaked to media for release on Tuesday - this has not yet occurred.

"It is inappropriate for employment disputes to be litigated in public," Smith said.

"I will put on the record that I regret the incident, I apologised at the time and I apologise again today.

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

It is understood the altercation involving Smith was with a young staff member who had worked there for less than a year prior to the incident.

It followed Parliament's review into bullying and harassment in the workplace – the Francis Review – which was completed in 2019 and led to Parliament adopting a code of conduct for MPs and Parliamentary managers and staffers.

Reti declined to answer any questions around Smith's resignation, including any details about the allegations, nor if it was appropriate for him to stay on through the election.

The party did not have a problem with bullying, and he did not know of any other similar instances involving MPs and their staff, he said.

There had been changes made since the Francis Review and this work was "ongoing", Reti said.

Reti had spoken with Smith, but this was "a private matter", he said.

He did not know if Smith was on sick leave or annual leave this week, nor if he would return to Parliament before June 10.

In recent years other National MPs have been accused of bullying staff in their offices, including Maggie Barry who was twice investigated during 2018 for alleged bullying.

Several National MPs denied the party had a bullying problem when questioned on Tuesday.

Party senior whip Matt Doocey said Smith was a "very loyal and hardworking member of caucus".

He declined to comment on the inquiry, but said "we expect a high standard of relationship between MPs and staff, a high level of respect and professionalism".

"This is an individual employment issue. He has owned his behaviour and I acknowledge that."

Doocey said he was "100 per cent" confident this was a once-off occurrence.

"We have a very good culture in the National Party," Doocey said.

Deputy Prime Minister Grant Robertson said the parliamentary workplace was a high-stress environment.

"It's fair to say we all know we need to do better. I'm not going to comment specifically on Dr Smith's case, but what the Francis review told us was we need to improve the workplace culture. That's never going to happen overnight, and clearly we've still got a lot of work to do.

"It is a difficult environment to work in, and it means we've got to work double-time to provide a safe work environment."

Leader of the House Chris Hipkins said the employment dispute was a matter for the National Party.

But on Smith, he said: "He's Parliament's longest continuously serving MP, and does deserve to be acknowledged for that."

Smith said he had decided to retire after losing the Nelson electorate in 2020, but that inquiry had prompted him to leave now.

Smith's resignation brings an end to a 30-year parliamentary career - a stint in which he was a Cabinet minister under Jim Bolger, Dame Jenny Shipley and Sir John Key, including Education Minister, Building and Construction Minister and Conservation Minister.

He was also briefly deputy leader of the National Party in late 2003.

The next candidate on National's list to enter Parliament is Harete Hipango.

Hipango has confirmed to the Herald that she will take up the opportunity to return to Parliament to replace Smith.

That will mean National has three Māori MPs – Reti, Simon Bridges and Hipango.