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.
A spokesman for the National Party said National was made aware of an alleged incident that took place at a separate location, after the conclusion of a Young Nationals event last week.
"The person who brought the issue to our attention gave no further details, or names of those persons involved.
"In line with our health and safety policies, we acted quickly to offer any support and advice to the person who raised the issue, and this information was passed on to the alleged victim.
"As this is now a matter for the police, we can't comment any further."
This alleged incident comes as National conducts an internal review into its culture, which is expected to be completed before the end of the year.
"We are getting independent advice to make sure we have got the best systems and process so women do feel safe," National leader Simon Bridges said when he launched the review last month.
The alleged incident comes after a police investigation into allegations of sexual assault at a Labour Party summer youth camp in February.
A man, who was charged with four counts of indecent assault, appeared in the Auckland District Court.
