A birthday for two 18-year-olds and a 13-year-old girl descended into a wild melee that has left a 17-year-old youth in hospital with a stab wound to his chest.
The party in Merrylands in the city's west was overtaken by gatecrashers, despite the best efforts of the mother to wind the party down from about 10pm and reports the father tried to keep the partygoers locked in the hall.
The party was held for the 18th birthdays of two friends, including one known by his Facebook name Canzo Tha Vatto, and the 13th birthday of Canzo's friend's younger sister.
One guest of the party described the events that led to the brawl saying that "There was about 100 to 120 people invited - and there were some people who came that weren't invited, but the guys knew them so it was OK - Then a whole lot of people just rocked up."
Asked what started the brawl, the partygoer said: "There was no main reason, there was just too many different groups of boys there and everyone had had too much to drink." The partygoer confirmed the youth that had been stabbed was seen as a gatecrasher by other guests to the party.
But as soon as the fight started, it quickly escalated.
"What made it worse was that our other mate got hit in the face and was bleeding - so as soon as everyone saw the blood on his face and Canzo's stomach... all-in brawl. As soon as they saw that the boys were hurt, everyone ran in. "
When asked if this attack would draw any kind of retribution, the source said: "[It] might, but to be honest I don't know. Last night I heard a lot of talk - the boys do know a lot of people, at least someone is going to want to go after them boys".
It is believed the tension between the groups of boys was due in some part to their geographical allegiances.
"There's been a bit of beef between the different areas, as there always is - there was a lot of Parramatta boys and Greystanes boys there - we're from Merrylands so we're kind of neutral. "
Responses to the stabbing lit up the Facebook pages of the two 18-year-olds, with one attendee commenting: "I wish parties these days weren't about fighting and doing stupid shit."
The brawl erupted after gatecrashers descended on the party, held at the Merrylands Park function hall.
When police arrived at 11pm, the brawl was already in progress. A male was arrested after a police officer was hit on the arm with a bottle while trying to break up the fight. The officer was treated in hospital and later released.
A partygoer said Canzo had been "sliced" in the brawl but did not require any treatment at hospital.
One Facebook post to Canzo read: "Hope you recover soon braadaa."
Earlier, police said they were not sure whether the youth who was stabbed in his chest had been invited to the function or turned up unannounced.
He remained in a serious but stable condition in Westmead Hospital early on Sunday morning.
Facebook posts after the party also suggested there was willingness to respond to the incident, with one post stating: "Where [sic] getting them back lad no matter what."
The mother of a girl who attended the party said between 50 and 70 invited guests attended. No security was hired for the event.
"When my husband went down to pick her up, there were a lot of groups of young people hanging out in the park area," the mother said, adding that the teenagers were doing burnouts in their cars.
The mother said her daughter was collected from the party about 10pm, before the brawl broke out, as the mother of the children celebrating their birthdays had begun advising the younger children to go home.
"My daughter said a lot of people were just showing up. A lot of people the family didn't know," she said.
Fairfax Media was told by someone with knowledge of the party that the children's father locked the door of the function hall to protect the guests while they waited for police to arrive. She claimed none of the guests of the party were involved in the brawl that took place outside the hall.
The mother was "an absolute mess", the mother of the girl who attended the party said.
"The family are very secluded, very quiet sort of people. This would be absolutely mortifying for them," she said.
The hall manager, who was not present on the evening of the brawl, has declined to comment.