National MP Sam Uffindell said he was a bully and "a 16-year-old thug" while in high school.

Uffindell revealed his bullying behaviour culminated in an incident that took place at the end of his fifth form year when he and others "raided" third-formers and he was asked to leave Kings College the following day.

"I've felt remorse and very upset about this for some time."

He said it had changed his life, and had an impact on the person he had hit.

"I'm not proud at all. I was effectively a bully. I was a mean person. There will be other people at high school that I have hurt.

"I might have tackled a few people, or punched a few people. As I said, I was a bully at school and I'm not happy with that.

He said he had also called people names.

Asked what made him think he could now have a successful career in politics, he said he worked hard and had empathy - he was very different from a "16-year-old who was a thug, quite frankly."

He said it had been damaging both for himself and the party.

"[The assault on a third-former] is by far the worst incident I was involved in at school."

He said he apologised to those people as well. He said he had since become an adult, got married and had children, and had thought about his own actions as a teenager.

"I'm a long way from that person I was 20-odd years ago."

Uffindell said his apology to his victim was sincere at the time and remained so today.

"I've grown a tremendous amount as a person.

"At high school, I wasn't a great person, There will be other people I have hurt. I've looked back over my years and said to my wife, and my parents - especially my Mum - that I don't like the person I was as a teenager."

Asked about the last 24 hours, he said they had been "awful, frankly."

"Really upsetting for myself and my family. But you know what, it's self-inflicted."

He said as an adult, he was able to empathise now, and so he had made the move to apologise to the man he had beaten.

He did not feel let down by the party, saying he had told them about the incident and they had seen his remorse.

He said the onus was on him to disclose it at pre-selection, and it was up to the party to decide how to handle it from there.

He admitted it was "an awful look" for him to have campaigned on a platform of law and order, given this incident.

Uffindell had used his maiden speech to Parliament to champion law and order – asked today if he was a hypocrite, he said: "It's an awful look, all I can do is apologise for what I've done."

Asked why he hadn't come clean earlier, he said in hindsight that should have happened.

He said he could have been "more forthcoming" about the incident during the Tauranga byelection campaign that he won.

The party had made the decision not to, and he said perhaps they should have talked about it together more.

"I was a 16-year-old and I made a big mistake. I've got young children and one day they will be at school, and if stuff like this happened I would be very upset."

He said the nature of his departure from King's College was disclosed to his next school, St Paul's Collegiate in Hamilton.

"You can't blame King's at all. It's for me to own, it's not an environmental thing."

National Party leader Christopher Luxon said he was standing by Uffindell, 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, Stuff reported on Monday.

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.

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.