Two males, aged 17 and 18 were arrested at a home in Eagle Vale on Sunday after police executed a search warrant.
Australian media report the fight broke out at an Ingleburn home before the brawl spilled on to the road.
The crowd grew bigger as more people arrived in vehicles, bringing with them a number of weapons including a baseball bat, golf club and wooden and metal poles.
Eighteen-year-old Alex Ioane, who was said to be attending a 16th birthday party, was discovered by emergency services on the road with serious head injuries.
