"I was a bully at school and I'm not proud of it," he said during a media stand-up today.
On Monday it was revealed that Uffindell was kicked out of his boarding school as a 16-year-old for beating a younger pupil.
He said he punched a 13-year-old boy in the arm and body "multiple times" when he was a pupil at King's College.
Uffindell said he was expelled at the end of his fifth form year, and was asked to leave the school the day after hitting the other boy.
It was a significant moment in his life that had a strong impact on him, he said.
He said that looking back, he doesn't really like the person that he was at that time.
He apologised to anyone he had hurt. He said he had grown a lot as an adult and had reached out to the victim to apologise.
"I'm a long way from that person that I was 20 odd years ago."
He said the last 24 hours had been awful - upsetting for himself and his family - "but you know what, they're self-inflicted".
He said he took ownership of the incident, had reflected on it over the years and wanted to atone for it.
"It had a significant impact on my life ... part of being a mature adult I'm able to empathise now."
He said it was a genuine apology.
Having made a speech on law and order and violence as his maiden speech at Parliament, he said it wasn't a great look and didn't help the cause.
"I've said to my wife and my parents - my mum especially - I don't like that person that I was back then."
He said the school acted appropriately in asking him to leave.
Asked why he didn't come forward during the by-election for Tauranga, he said in hindsight he should have come forward about it.
He said he didn't recall whether there was a discussion with the party about whether the incident should have been disclosed during the campaign.
"Look, I was a 16-year-old, and I made a big mistake, you know. I've got young children and one day they will be at school and if stuff like this happens I will be very upset."
He acknowledges the assault was unlawful.
He had a conversation with the preselection committee about the incident, he said, and then-party president Peter Goodfellow would have been aware of it.
Asked if he feels he came to Parliament dishonestly, he said he gave a full written and verbal statement to the party.
The voters should "potentially" have been informed about it, but he said he is a significantly different person now than he was when he was 16.
"In hindsight, we would have been better to get that out earlier."
He said he was sure he had in the past hurt people both physically and mentally, and was a bully when he was at high school.
"It makes me reflect a lot on what school was like and who I am as a person ... and the example I want to be setting to people."
This was by far the worst incident he was involved in at school, he said.
Earlier this morning, Uffindell told RNZ there was "rough and tumble" when at the boarding school but the attack on a student was the most serious incident he was involved in "by a long way".
Uffindell apologised to the victim last year, nine months before he announced his candidacy in the Tauranga by-election, but said in his interview on Morning Report there was no link with wanting to launch his political career.
'He has my backing and he has my support' - Luxon
Meanwhile, National leader Christopher Luxon is standing by the MP for Tauranga, but says the assault of a smaller boy at boarding school should have been made public.
Fronting the media at Parliament this morning, Luxon said he had been unaware of Uffindell's assault on the boy until yesterday.
"He has my backing and he has my support but clearly he needs to be able to build back trust with the voters in Tauranga, and make sure that he is ... demonstrating as we go into the next election - that he's a person of good character."
He said Uffindell had declared the details of the incident when he sought the candidacy, and there had been a thorough process including seeking comment from third-party references, but a few of things should have happened that did not.
"The assessment was 'hey listen, no, Sam is a changed individual from the 16-year-old we saw 22 years ago ... so that I think did work well, because at least the discussion was had, there was deep exploration of the issue.
"The thing that could have been different, frankly, is that I should have been informed rather than finding out yesterday, the second thing is the delegates ultimately should have been informed.
"Most importantly I think the voters in Tauranga should have been informed so they can form their own judgement and should have had that information in the campaign."
He said Uffindell was genuinely remorseful and regretful and had fronted up on the matter.
"He's fronting and he's owning that. And I expect him to do that."
"There is no place for violence in New Zealand - no setting, no context, no excuses - and we abhor it and we don't condone it whatsoever."
The selection process for candidates includes an initial screening with reference checking, then a selection panel, Luxon said.
"All those people were fully aware as they weigh up a series of candidates and they can choose to let them go through or not go through and there was deep exploration of this issue with Sam."
After that, the selection goes to 60 delegates who are party members, who each candidate spends time with.
