Man charged over wild Sydney house party

An 18-year-old who allegedly doused an ambulance with petrol at a Sydney house party is set to face court later this month.

Police say paramedics had been helping a 14-year-old boy who was burned while trying to light a bonfire at the Rossmore party late on Saturday night, when a drunken group began misbehaving and blocked access to the fire.

He suffered superficial burns to his face and hands.

A paramedic returned to his ambulance an hour later to discover it had been doused in petrol, says Acting Inspector Branko Kubecka from Green Valley Local Area Command.

"(He) noticed the entire van was wet and he smelt the strong smell of fuel," he told AAP on Sunday.

An 18-year-old charged with causing malicious damage and was granted conditional bail to face Liverpool Local Court on May 26.

"He was two hours into becoming an adult ... he obviously made some pretty dumb decisions - alcohol was a contributing factor," Act Insp Kubecka said.

It's believed the drunken group who were blocking access to the bonfire weren't aware of their actions.

"I don't think they were intentionally impeding access," he said.

"I think they were probably oblivious to what was going on."