It comes after Stuff revealed on Monday he had beaten a student three years his junior while at King's College in 1999.
Uffindell yesterday admitted he was a "bully" and a "thug" in his youth.
Luxon reiterated how the new allegations were "deeply concerning and needed to be treated "very seriously".
An investigation into the latest allegations would be conducted by Maria Dew QC and is expected to take two weeks.
When asked whether Uffindell should "fall on his sword", Luxon only referenced the allegations and how he wanted them addressed in an appropriate manner.
Luxon defended campaign chairman McClay, saying McClay informed Luxon's staff of the King's College incident after it was disclosed by Uffindell but in a "genuine mistake", the staffer did not pass it on to the party leader.
"That is regrettable and it's a mistake," Luxon said.
"But I have to say, I trust my staff."
Uffindell has denied any accusations he was involved in bullying or intimidatory behaviour while at university.
Ardern and Luxon go head-to-head
Meanwhile, Luxon and Prime Minister Jacinda Ardern faced off during Question Time on the topic of social welfare after National's policy launch over the weekend.
Ardern sought to hit back at National's assertion the Government was giving young people a "free ride".
Luxon challenged Ardern on the fact more than 50,000 more people are on Jobseeker benefits now than when Labour came into government in 2017.
Ardern said a "record" number of young people were coming off benefits and into employment with over 35,000 people aged 18 to 24 doing so last year.
Luxon also challenged Ardern on the use of sanctions designed to encourage people into work, which had decreased dramatically while benefit numbers increased.
He pointed out sanctions were now 50 per cent lower in the June quarter than in 2019, while benefit numbers were 25 per cent higher.
Ardern said people could still be meeting their obligations while also not being ready to enter the workforce.
"What we have is an economic crisis. And in the wake of the [Global Financial Crisis] we saw a similar increase for young people impacted and on benefit.
"When I see a reduction in sanctions, my assumption is it's because young people are fulfilling their obligation rather than the National Party's assumption that young people are suddenly being given a free ride."
Earlier developments
This morning Luxon told Newstalk ZB's Mike Hosking the situation was very concerning.
"You got a situation where a young woman, a flatmate has made serious allegations. It is a concerning situation for a father whose daughter is flatting, it's very real. They are serious allegations that need to be investigated. Sam is disputing the allegations."
Asked if Uffindell still had his backing, Luxon told TVNZ: "We'll find out in the next few weeks because really, what this investigation is about, is looking at both sides of that investigation."
Luxon also acknowledged that the investigation was needed to honour the "alleged victim" too.
National Party president Sylvia Wood said the investigation into the latest allegations would be conducted by Maria Dew QC and is expected to take two weeks.
The new allegations came a day after revelations that the new Tauranga MP, as a teenager, beat up a younger boy at boarding school.
The revelations have also raised questions about National Party culture and selection process for MPs. In recent years several National MPs and candidates have been embroiled in controversy.
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.
Luxon said National's character checking extended to speaking to people who had known Uffindell since after the King's College beating.
