Police were called to a hall in Sydney's south after about 30 teenagers gatecrashed a 16th-birthday party where one man was injured.
About 9pm on Friday, fights broke out between partygoers and gatecrashers on Queens Road, Hurstville.
One gatecrasher allegedly took out a knife and assaulted a 17-year-old man, causing a scratch to his elbow.
Paramedics treated the man at the hall and another man was treated after he stood on glass.
About five cars in the car park were damaged by the gatecrashers before they were moved on by police.
Two men were detained at the hall and were given criminal infringements notices. Police took another intoxicated man home.
St George Local Command police will take statements on Saturday and are searching for the man who allegedly wielded the knife.
The man has been described as of Pacific Islander appearance, aged in his teens with a lanky build.
He was wearing a 'G-Star' brand hooded jumper, a green bomber jacket, beige coloured chinos and a green hat.
Anyone with information is urged to call Crime Stoppers on 1800 333 000.
