A teenager has died following a brawl at a 16th birthday party in Ingleburn on Friday night.
Police say the brawl began at an Chester Road home before spilling out on the street, with more people arriving armed in vehicles, triggering a full-scale emergency response about 9.40pm.
Police seized a number of weapons - including a baseball bat, golf club and metal poles - after an 18-year-old man was found on the road outside an Ingleburn home unconscious and suffering serious head injuries.
He was unable to revived and was pronounced dead at the scene.
Detective Chief Inspector Michael Moroney said between 50 and 80 people attended the house party and police arrived to a "chaotic scene".
"This is very tragic set of circumstances. We've got a young, 18-year-old boy who goes out for the evening and has been killed in such a brutal manner," Inspector Moroney said.
A friend of the 18-year-old man said there was a fight inside that escalated onto the street.
Asked what it was over, the woman said, "something petty, something little that happened inside the party".
She described her friend as a well-loved person who cared about others.
"He was bubbly, he was funny, he was always happy," the woman said.
The NSW Police Riot Squad, the Dog Unit, the Rescue Squad as well as Campbelltown police descended on the address, with crowds dispersing after police arrived.
Several crime scenes were set up as homicide detective investigate the circumstances surrounding the man's death.
Inspector Moroney said the cause of the man's death was yet to be determined and that he didn't believe anyone else was injured during the fracas.
"This was a targeted-type incident, it wasn't a random attack, there was obviously something that has occurred at that party, the nature of which will form part of the investigation," he said.
Four men, all aged 19, were arrested at the scene of the brawl and taken to Campbelltown Police Station for questioning.
Two men have since been released and no charges were laid.
The friend of the deceased teenager said the fight was unexpected and "something you don't want to remember, to be honest."
"We want justice... and for the person who did this to be behind bars," she said.
Inspector Moroney appealed for anyone who was in the area between 9 and 9.40pm, particularly those who may have filmed or taken photographs, to come forward "as a matter of urgency".
Investigators are urging anyone with information about the brawl to contact Crime Stoppers on 1800 333 000 or crimestoppers.com.au.