National MP Sam Uffindell has been stood down from the party's caucus while an investigation is carried out into further allegations of bullying raised by RNZ.

A woman who flatted with the Tauranga MP at university in 2003 has told RNZ's Morning Report Uffindell was an aggressive bully who once pounded on her bedroom door, screaming obscenities, until she fled through her window.

In a statement issued late Tuesday night, Uffindell denied claims he engaged in "intimidatory or bullying" behaviour, but said there was a falling-out between flatmates.

National leader Christopher Luxon released a separate statement, describing the accusations as "very concerning".

"Mr Uffindell disputes the allegations and in the interests of natural justice, an independent investigation will now be undertaken to determine the facts. While this process is underway, Mr Uffindell will be stood down from caucus.

"The investigation will be conducted by Maria Dew QC and I expect it to take two weeks.

"However, as these allegations have only come to me in the last few hours, the finer details of the investigation, including the terms of reference, are yet to be confirmed and will be finalised over the next couple of days."

Uffindell's former flatmate, who RNZ has agreed not to name, lived with the man and three other Otago University students for several months in Dunedin in 2003.

She told RNZ Uffindell engaged in a pattern of bullying during their second year at university, describing him as "verbally aggressive".

Uffindell would trash the house after "excessive" use of alcohol and drugs, she says.

"This was intimidation. This was bullying. I didn't feel safe," she says.

The woman says she eventually moved out of the flat after having to lock herself in her bedroom to avoid a drunken outburst one night.

"He was smashing on my door and yelling obscenities and basically telling me to get out - 'hit the road, fatty'.

"I ended up climbing out of my bedroom window and ran to a friend's house to stay the night. I feared for my safety. I was scared."

The woman says it was not an isolated incident: "it was just the straw that broke the camel's back." Her father travelled to Dunedin the next day to help her move out, she said.

Speaking to RNZ, the woman's father corroborated his part in the story and says his daughter had been "seriously upset".

"The flat itself was completely trashed. There wasn't a stick of furniture left. There was no crockery left. There were no handles left on anything. It had all been broken."

He says he gave Uffindell and two of the other flatmates "a serious piece of [his] mind" at the time.

"It was clear... [Uffindell] had real issues, real problems... he was out of control."

The woman says she was traumatised by the event and did her best to avoid Uffindell from then on: "my stomach would absolutely flip and drop if I saw him".

Looking back, the woman says she should have spoken to someone or taken some sort of action, but she was too scared. Uffindell never apologised for his actions, she says.

She says people may try to excuse Uffindell's actions because of his age, but the pattern of behaviour revealed his character.

"Listening to his maiden speech in Parliament, he talks about lack of accountability and a sense of impunity - I think that's so hypocritical."

Uffindell responds

In his statement, Uffindell says he "enjoyed a student lifestyle" at university including drinking and smoking cannabis, but the bullying allegations "simply did not happen".

"While in second year a number of flatmates fell out - and two of the flatmates left midway through the year," he says.

"I reject any accusation that I engaged in behaviour that was intimidatory or bullying. This simply did not happen.

"While there is an investigation into these accusations I will not make further comment."

New National president Sylvia Wood says she was advised Tuesday evening of the developments and the party would finalise the details of the investigation over the next few days.

The development comes after Stuff earlier revealed Uffindell, as a teenager, beat up a younger boy at Auckland's prestigious King's College boarding school.

Uffindell on Tuesday described himself as a "16-year-old thug" but says he is now a changed person.

He says the night-time attack was "by far the worst incident" he was involved in at school.

-RNZ/Craig McCulloch.