A teenager has been charged over an attack on another boy at a gatecrashed Halloween party which left him seriously injured in hospital.
The 17-year-old from Ryde was arrested at home and charged with grievous bodily harm after the 16-year-old was punched at the gathering in Gladesville about 11.30pm on Saturday.
The blow caused him to fall to the road and hit his head. He was found unconscious by ambulance staff and underwent surgery on Monday.
The boy's condition has since improved to serious, according to staff at Royal North Shore Hospital. However police say the teenager remains in an induced coma.
AAP has been told detectives seeking information on why the fight broke out believe a number of people who could help their investigation are yet to come forward.
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They remain unsure whether the teen was invited to the party or turned up unannounced.
The accused 17-year-old will appear in the Children's Court on Tuesday.
A day before the attack, about 60 intruders gatecrashed a Halloween party at Milperra.
Police were called about 9.30pm on Friday after the gathering of about 20 was overrun.
A man believed to be in his late 50s and a second, younger man, were struck in the head several times after a brawl erupted.
The younger man was taken to Bankstown Hospital with lacerations to his face.
Riot Squad officers helped disperse the gatecrashers who reportedly found out about the party via social media.
Police have also cautioned anyone throwing house parties against supplying alcohol to minors.
They say it's not only illegal but potentially "very dangerous".
