Gatecrashers brought a sawn-off firearm to Heath Donoghoe's birthday party, threatened guests, bashed a partygoer and are being hunted by police.
Friends were celebrating Mr Donoghoe's 21st birthday on Saturday night when a group of uninvited men arrived.
The group smashed a front window and assaulted a young man, breaking his nose and eye socket before running off.
Mr Donoghoe said he did not know the men or why they turned up.
"They must have known someone here. We didn't know who they were," he said.
He said one of them had a shortened firearm, which police later found in the bushes over the back fence of Mr Donoghoe's home.
The gun has been seized for forensic analysis.
Police swarmed on the home in Fletcher Street, Adamstown, about 9.30pm.
"I've never seen so many police," Mr Donoghoe said.
Mr Donoghoe and his mates were undeterred by the incident .
Detectives have taken statements from partygoers in an attempt to identify those involved.
Police said the gatecrashers were all Pacific Islander males.
Anyone with information should contact Crime Stoppers on 1800 333 000.