The future of National Party Tauranga MP Sam Uffindell won't be clear for some weeks as the investigation into historical allegations of bullying against the MP has been extended.

It is understood the independent investigation, led by Maria Dew QC, might not conclude until mid-September.

Uffindell, who won Tauranga's byelection earlier this year, was stood down from National's caucus on August 9 after two allegations of bullying and intimidatory behaviour in his youth surfaced.

On August 9, National leader Christopher Luxon said he expected the investigation would take two weeks.

He later said the investigation would start on August 15, and re-stated his expectation of a two-week duration.

In the latest update, Dew had reportedly requested more time to complete the investigation.

National Party leader Christopher Luxon initially expected the investigation to be completed in two weeks. Photo / Mike Scott

The NZ Herald has requested comment from the National Party to explain the reasons behind the extension.

On August 8, Stuff revealed Uffindell had been asked to leave Auckland boarding school King's College for physically beating a Year 9 (third form) student while Uffindell was Year 11 (fifth form) in 1999.

• 'Throwing stones into glass houses': Uffindell's uni rugby mates defend 'good dude'
• 'Old boys club': Former King's students speak on culture at school
• 'Voters should have been told' - Nat leader supports MP after bullying revelation

It reportedly left the victim with severe bruising and significant trauma.

"It was one of the silliest, stupidest things I've ever done. I really regretted it, I do really regret it still," Uffindell said at the time.

Following that, RNZ revealed another set of allegations of bullying and intimidatory behaviour from a former flatmate of Uffindell while the pair were living in Dunedin while attending university.

The woman claimed Uffindell was an aggressive bully who once pounded on her bedroom door, screaming obscenities, until she fled through her window.

National Tauranga MP Sam Uffindell with leader Christopher Luxon in June. Photo / Mark Mitchell

Uffindell disputed her allegations.

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

In a statement late on August 9, Luxon confirmed Uffindell would be stood down and an investigation would take place.

"This evening my office became aware of very concerning accusations made to RNZ about behaviour shown by Mr Uffindell toward a female flatmate in 2003 while at university," Luxon's statement read.

"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 under way, Mr Uffindell will be stood down from caucus."

Uffindell dominated the political news cycle until he was trumped by Labour MP Dr Gaurav Sharma, who on August 11 published an explosive column in the Herald claiming bullying within his own party.

The claims, which the Hamilton West MP repeatedly aired publicly against the wishes of Labour, led to his expulsion from the party's caucus and he is now an independent MP.