A teenager has been charged after he allegedly brandished a fake gun at an end of school party in Sydney's south.

The 18-year-old is accused of throwing a glass bottle at two 16-year-olds and threatening them with a replica pistol at a party in Sylvania Waters in the early hours of Saturday.

He is also accused of assaulting the two teenagers and another man, as well as trashing the front door of the home and damaging a side mirror of a nearby car.

Police on Sunday arrested an 18-year-old man at his Miranda home, where they say they found a fake gun and some cannabis.

He was refused bail and will appear at Sutherland Local Court on Monday on charges including assault, intimidation and possessing an unauthorised prohibited firearm.