National Party leader Christopher Luxon is standing by MP Sam Uffindell after he assaulted a schoolboy as a teenager, but says he should have been told of the incident earlier.

"He has my backing and he has my support but clearly he needs to build back trust with the voters of Tauranga," Luxon said.

Aged 16 as a Year 11 student at King's College, Uffindell and three others jumped on the then 13-year-old boy and began beating him with what was believed to be unscrewed wooden bed legs.

Luxon confirmed Uffindell had declared the incident to the National Party when he sought to be a candidate.

"He is not the same person that he was 22 years ago as a 16-year-old."

Luxon said he should have been informed earlier. The delegates should have been informed and the voters of Tauranga should have been informed earlier.

He said Uffindell's admission during selection had triggered deeper background checks and he believed that was how it was supposed to work.

"There was a deep exploration of this issue with Sam."

Luxon said it was important that the new MP was fronting up to explain the incident "and he's owning that".

He had been assured that Uffindell did not have any other incidents in his past.

• Political commentator: National is in for some tough questions about Uffindell

Luxon said National's character checking extended to speaking to people who had known Uffindell since after the King's College incident. He did not know if King's College itself was contacted as part of that process, but stressed that Uffindell had told the party about it.

The violence committed at the school was "totally unacceptable and abhorrent", Luxon said.

Clearly the victim had suffered and the attack had caused a lot of "pain, hurt and suffering" for the victim, Luxon said.

"I see it as assault, I absolutely do."

Luxon said Uffindell was now a person of "high integrity" and that a good MP "deserves a second chance".

The coverage of Uffindell's bullying history had been "immensely frustrating" for National, but it was important for the party to confront it.

New President Sylvia Wood was on the selection panel - Luxon said he did have confidence in her.

National MP Sam Uffindell said there was a culture of "rough and tumble" when asked if he had been involved in any other incidents of bullying as a schoolboy.

"Boarding houses in the 1999, there was a bit of rough and tumble that went on... we would tackle and punch each other around a bit, but I wouldn't go out and focus on someone.

"This was the most serious," he told RNZ.

He also said that any other incidents were "not targeted" but would not disclose details.

Uffindell reiterated that when we was asked to leave King's College it was solely related to the assault he was involved in made on a 13-year-old student.

He said the incident was his fault and he takes full accountability.

"I try and live my life as an adult as a responsible person to set a good example to my children and to others."

The National Party leader Christopher Luxon and his deputy Nicola Willis only learned of Sam Uffindell's school teenage dormitory assault yesterday afternoon, it has been revealed.

Willis told RNZ said she found out about the incident just after lunchtime and she believed that Luxon also found out then.

She said Uffindell had disclosed the incident to the pre-selection panel, made up of local and national party representatives, and they had made the decision about not precluding him from standing for Parliament.

"That's a party matter. That's their judgement. Where I stand on this today is that I have advised Sam that what he should do now is be completely upfront with New Zealanders about this because ultimately it is the people of New Zealand and the people of Tauranga who will be the judges of us."

Willis described it as a serious incident and her thoughts were with the victim, saying it would have been a traumatic event at the time and something that would never leave you.

She said there should be room in Parliament for those who made serious errors, accounted for them but were now committed to using their position for good.

"If I thought that Sam was still the same man as he was when he was a 16-year-old when he committed this act then I don't think there would be a place for him in Parliament. However, I see that he is extremely sincere in his regret in his genuine apology and that he is being upfront about what occurred and that he is a different person today than when this happened."

His former school, King's College, has also spoken out after it was revealed the National MP assaulted a 13-year-old student while attending the school.

King's College headmaster Simon Lamb confirmed the incident and said it was dealt with at the time.

"The issue referred to in the Stuff article today was a matter which the College dealt with 22 years ago," said Lamb.

"Since that time, the College has not been involved in any follow-up activity with those involved, including the recent discussions reported in the article."

Sam Uffindell is the MP for Tauranga. Photo / File

The incident reportedly occurred in 1999 on the last night of term inside one of the King's College boarding houses.

It was reported by Stuff that the now MP for Tauranga apologised to his victim 22 years after the attack and nine months before he revealed his political aspirations.

Uffindell has detailed the late-night violent beating of a younger boy that led to him being asked to leave King's College while he was a student.

Speaking to Newstalk ZB's Heather du Plessis-Allan, Uffindell confirmed the incident and said it was "one of the dumbest, stupidest things I have ever done".

He also wouldn't rule out standing down as MP.

"It was one of the silliest, stupidest things I've ever done. I really regretted it, I do really regret it still," Uffindell told Newstalk ZB.

Police were not involved but he was asked to leave King's College, and finished his schooling at St Paul's Collegiate in Hamilton.

Uffindell said at the end of the school year students went into the third-form dorm and "raided them".

With the boy, he said he punched him a "bunch of times" in the arms and body.

He and the other boys were called into the school the next day and asked to leave, Uffindell said.

He said he was gutted about the incident, "had taken ownership of it" and had thought about it for years.

He said he had "no recollection" of using bed legs to beat the boy.

"I still am sorry for what happened, I wish it had not happened."

A statement from the National Party said the party had been "proactively informed" about the incident by Uffindell during the selection process ahead of the Tauranga byelection.

"It was a significant event reflecting a serious error of judgment by a then 16-year-old for which he has apologised and regrets to this day," the statement read.

The victim, who was not named, told Stuff that Uffindell contacted him through a mutual acquaintance in July last year to apologise, which the victim eventually accepted.

"But then a few months later I sat down to watch the news on the couch with a beer and there he was, running for Parliament," the victim said. "I felt sick."

Uffindell had reportedly not mentioned his political intentions during the interaction.

"We had a long conversation and I was grateful that he accepted my apology. My apology was genuine then, and it is genuine now."