National MP Sam Uffindell says there was "rough and tumble" when he was at King's College but the attack on a student was the most serious incident he was involved in "by a long way".
The party's newest MP has admitted he was kicked out of his boarding school as a teenager for beating a younger student.
In a statement, Uffindell said he punched a 13-year-old boy in the arm and body "multiple times" when he was a student at King's College.
"I made a big mistake," he told Morning Report.
"I was 16 and we raided the dorm of the third formers on the last night of the year and punched the victim a number of times in the arm and body and he was subsequently quite hurt from it and the school rightfully asked me to leave.
"I deeply regret, and have since, my involvement in that and the impact it had on the person."
He said the attack which saw him asked to leave King's College was the most serious incident he had been involved in.
He had not been involved in any other targeted bullying though it was a "rough and tumble" environment, he said.
"Like a lot of people I partook in a bit of that. This [attack on the 13-year-old] is the most serious by a long way," he said.
"We would tackle and punch each other around a bit but I never really went out to focus on anyone in that regard.
"It was a rough and tumble environment and I wasn't away from that and sometimes I was on the receiving end of it, it was just how it was.
"But I'm not downplaying my part at all in this event at all, it was a serious event and one I'm deeply remorseful for having been involved in."
Uffindell apologised to the victim last year, nine months before he announced his candidacy in the Tauranga by-election, but said there was no link with wanting to launch his political career.
He had made the apology before knowing there would be a by-election due to former leader Simon Bridges' resignation, he said.
"I carried this as a burden and I wanted to atone for it.
"I wanted to talk with him, I wanted to apologise and see how he felt and just see if I could work with him to get some closure on what was a significant event for both of us."
National Party stands by Uffindell
National Party deputy leader Nicola Willis acknowledged the seriousness of the incident and told First Up her thoughts were with the victim.
But she said Uffindell has changed and is sincere in his regret.
"If I thought that Sam was still the same man as he was when he was a 16 year old who 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 he has been upfront about what occurred.
"I think there has to be room in Parliament for people who have made serious errors, accounted for them and who are now committed to using their position for good."
Willis said both she and leader Christopher Luxon found out about the assault just after lunchtime on Monday.
The party has confirmed the incident was raised during the candidate selection process, saying it reflects a "serious error of judgement" by a then-teenager.
The panel was made up of local and nation party members, Willis said.
It decided he shouldn't be precluded as he had accounted for the actions, was 16 at the time and a different person now, and was apologetic, she said.
"That's a party matter, that's their judgement and 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 on this."
"I'm conscious here that we are talking about very serious behaviour by a 16-year-old and as a teenager, many of us would have made serious mistakes."
The idea of her 12-year-old son "being subjected to bullying of this nature is deeply upsetting", she said.
There was an opportunity for everyone in Parliament to ensure they use their positions to "say no to this kind of thing" and to be leaders who condemn it.
"I fully expect that Sam will join me in that condemnation."
The party wouldn't be distracted by these issues, she said.
Asked by RNZ if he will be staying on as an MP, Uffindell said he would be having talks today in Wellington about his future.