Accused one-punch Halloween party gatecrasher in court

A gatecrasher was seeking a fight when he punched a boy in the back of the head and left him unconscious outside a Sydney house party, a court has been told.

The injured 16-year-old boy was placed in an induced coma in hospital after being king-hit outside a Gladesville home where a Halloween party was being hosted on Saturday night.

Paramedics found him unconscious after he hit his head on the road.

His alleged attacker, a 17-year-old boy, wept in Parramatta Children's Court on Tuesday after being refused bail.

His lawyer told the court she would enter a plea of not guilty.

She said the boy suffered from severe depression, wasn't attending work or school and was willing to meet multiple strict bail conditions.

However, the magistrate decided his unacceptable risk to the community couldn't be mitigated.

Police told the court the accused was part of a group that gatecrashed the party before he "attempted to provoke a fight".

It is alleged the victim had turned his back on the young person when he was hit.

"Those matters are what make this offence particularly serious," a prosecutor said.

Upon being refused bail, the boy stared at his father and began tearing up before placing his head in his hand.

He will be held in youth detention until his return to the same court on December 6.