The person wasn't comfortable approaching the investigator themselves.
In a press release yesterday afternoon, the firm said the board was in the process of finalising the appointment of a suitably qualified person, outside of the legal community, to undertake the review.
Russell McVeagh have also said the final recommendations would be shared with parties which the company had a professional obligation to, including the women involved, staff, clients, and the New Zealand Law Society.
Law Society president Kathryn Beck said that, and the review, was welcome.
"That's quite a brave commitment."
As well as potentially giving "real clarity" about what happened in the summer of 2015/16, the insights gained and recommendations made by the investigator could be useful across the industry in future.
"It's about culture change. People can have all the things on paper ... but it's about what happens in the workplace."
The review was announced following allegations by former Russell McVeagh staff members to Newsroom over sexually inappropriate behaviour.
A former employee told the Herald on Sunday senior managers at the leading law firm had ignored repeated warnings about the behaviour and the drinking culture.
Two incidents reportedly happened at Christmas functions and another at the El Horno Bar in Wellington. At least one complaint was made to police about a man's behaviour at El Horno.
Victoria University vice chancellor Grant Guilford wasn't available yesterday but previously said he understood a young law clerk student who was part of a summer programme made an allegation of sexual assault.
He was aware of several other young women who also allegedly experienced sexually inappropriate behaviour at the time.
Long-standing Russell McVeagh chief executive Gary McDiarmid said full internal investigations took place after the allegations were made, and the subject of the allegations left the firm.
The review would include all actions taken in response to the specific incidents, management practices and policies in relation to preventing sexual harassment, and supporting those who wished to make complaints, and the organisational culture of the firm.
What happened in the summer of 2015/16 was "completely unacceptable" and the "brave" women who spoke out about it at the time had led to actions being taken to improve the workplace culture, the firm said in yesterday's statement.
The firm believed actions taken immediately after becoming aware of the incidents were the right ones.
"Our ongoing approach to ensuring that no incidents of sexual harassment or bullying occur in our workplace is relentless and is supported by our partners and staff.
"We need to understand if, how, and where we made errors so that we can apply those learnings in the future in our management practice and embed them in our culture."
The reviewer would make recommendations for change and would have full access to all material and staff.
Current staff would also be expected to take part in the review. Former staff and summer clerks would also be invited to participate.
Meanwhile, amid the turmoil besetting the firm, McDiarmid is leaving.
He had announced to partners in June - on his 20th anniversary with the firm - that he was "planning on leaving within the next two years with a final date to be agreed with myself and the board", he said.
"In December I confirmed to partners that I would leave at the end of this year. The media around the events of the summer of 2015/16 are in no way connected to my departure."
