It is alleged a woman, aged 17, and some friends went to the young man's apartment after the event.
The woman was not a Young Nat member and was a guest at the event, according to Newsroom.
The woman was allegedly taken to a room at the man's apartment, where he tried to grab her face and kiss her, while keeping her away from her friends.
After leaving the room and the apartment, it is alleged she was pursued across the road and into a fast-food outlet's toilets by the man, who allegedly continued to grope her.
Speaking to media this morning, Goodfellow said: "We understand that they don't want us to know, and that's why we haven't been advised."
He said he found out about the alleged incident last week.
"I don't know who the victim is and I don't know any details about the victim."
He does not have any knowledge about the perpetrator either.
He said it's been a few days now, but the people involved in the alleged incident had not yet identified themselves.
"As soon as we were advised that there was an incident, we acted very quickly within our health and safety guidelines to offer support and advice to the person who advised us, who was an intermediary and not the alleged victim."
He said the matter was for the police.
Goodfellow said despite not knowing who the victim was, National was supporting them though this intermediary.
He said a complaint had not been lodged with National relating to the event: "We have just had advice of this."
Goodfellow said all National events were covered by its health and safety guidelines.
"I have checked and [with] this one, I am very satisfied, very confident that there was a proper plan [in place]," he said, stressing that the alleged incident did not happen at the Young Nats event.
He said the alleged perpetrator needed to have the opportunity to put their case forward.
"We're not the judge and jury – this is a matter for the police."
National leader Simon Bridges said the alleged incident was a matter for the party to handle.
He said he learned about it late last week and he sought assurances that everything had been handled and dealt with appropriately.
"Nothing untoward happened at the incident, it was post that."
Bridges said he was not concerned about any cultural issues within the Young Nats because "ultimately, the health and safety plan was followed".
A manager at Brew Bar on Quay, the venue where the Young Nats Christmas Party, did not want to comment on the the party.
Pakuranga MP Simeon Brown said he was at the party "very briefly and left early".
"I wasn't aware of anything that happened but understand there has been a complaint made and that's with the police."
He said he was not familiar with the details of the event, as he did not organise it.
Hunua MP Andrew Bayly was also at the event.
He said the party was "pretty quiet, there was plenty of food and plenty of adults … It was a very nice event, very low key," adding that no one at the event was intoxicated.
