He remains in a serious but stable condition at Auckland City Hospital. His family are with him.
Police are investigating the assault and will continue speaking with partygoers and other witnesses today.
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Read the Herald's earlier story below:
An eyewitness watched in horror as a gatecrasher being dragged from a party near Eden Park was seriously assaulted shortly after he left the Auckland Nines tournament.
The 23-year-old left the stadium with his brother and was walking along Bellwood Ave in Mt Eden when he came across the party.
A witness to the assault, which occurred about 8.15 on Saturday night, said partygoers began drinking throughout the day but there had been no trouble. However, tenants had been forced to shut the gates to keep uninvited people out - Eden Park is only about 250m away.
But people kept walking on to the property and neighbours heard a lot of "who the hell are you" type comments addressed to the uninvited guests.
The victim suffered head injuries and last night was in a serious but stable condition at Auckland City Hospital with his family at his side.
A witness described hearing the assault, "which didn't need to happen". It began with shouting, which turned into a scuffle and then a fight.
"I went outside and saw them trying to evict this guy from the premises by dragging him," the witness said. As this happened, punches had been thrown.
"He got beaten up quite badly. This guy was drunk, wanted to have a few beers, decided to gatecrash somewhere, meets people who don't want him there and a fight occurs.
"It's such a sad thing, both for the people who might have hurt this guy and the person who got hurt. It didn't need to happen," the witness said. "My thoughts go out to this poor guy ... I was totally horrified by what happened."
The incident was out of step with the "jovial atmosphere" around Eden Park. "What happened wasn't a reflection of what happened at the stadium or the Nines," the witness said.
Detective Inspector Chris Cahill said: "Staff have spoken to residents and partygoers and are trying to get to the bottom of what took place and who is responsible for the young man's head injuries.
"We are liaising with hospital staff and supporting the man's family at this difficult time."
A man who was at the party and knows the residents said it was supposed to be a quiet gathering.
"There were about 50 people or less, I would say. The party had been organised by the people living in the house and everyone invited their own friends, which was meant to be a fun [time] and not turning out this way," said the man, who spoke on the condition his name wasn't published.
"I was sitting in the back of the house with some friends and ... we rushed over and I could see the guy lying down on the ground ... Everyone was shocked by what happened."
He believed the victim and his brother weren't from Auckland. He has earlier spoken to the man injured.
"He wasn't any trouble and was a really friendly and good person and didn't deserve that at all," he said.
"I don't know if he was invited or not, all I know is that he knew one of the guys there, not the flatmate but a guy who was invited. It's possible he has been asked to leave the party."
Extra officers had been rostered on to help police the city as thousands attended the weekend's big events.
There were only a handful of arrests at Eden Park. Despite more than 100 people being checked by St John staff on site, no one was taken to hospital.
"Considering the number of people out and about over the weekend there's been little out of the ordinary for police to deal with," said spokeswoman Noreen Hegarty.
