A teenager has been charged after he allegedly doused an ambulance with petrol during his own 18th birthday party in Sydney's south-west.

Emergency crews were called to a home on Devonshire Road in Rossmore about 11:30pm yesterday after reports a 14-year-old boy had fallen into a bonfire.

Two ambulance vehicles arrived at the property and paramedics treated the teenager before he was taken to The Children's Hospital at Westmead with burns to his face and hands by one of the ambulances.

He remains there in a stable condition.

Green Valley Police Acting Inspector Branko Kubecka said when an ambulance officer arrived back at the property to pick up the remaining ambulance he found that petrol had been poured over it.

"There was no visible damage in terms of fire, but he noticed it was wet and smelling strongly of petrol," Acting Inspector Kubecka said.

"NSW Rural Fire Service was called and washed the car down, however, the strong smell of petrol remained.

"And inside the cabin, it will need to be detailed and cleaned properly."

He said police later arrested an 18-year-old male, whose birthday party it was, and charged him with causing damage to property.

Ambulance officers just doing their job

Acting Inspector Kubecka said the incident was alarming.

"The ambulance officers in their professional decision decided the best patient care was to make sure that they're all in the same ambulance, to make sure that the 14-year-old boy was treated correctly," he said.

"And in good faith left their vehicle and to return and find that it's been damaged, it's quite senseless."

He said everyone was entitled to have a few drinks, particularly on their own property but parents needed to take responsibility to make sure things did not go too far.

"You can have a good time without committing an offence like this," he said.

The Rural Fire Service were originally called to the property to attempt to extinguish the bonfire, but were forced to call police when partygoers became aggressive and refused to let them put out the blaze.

Green Valley Police were able to move the people back into the house while the fire was extinguished.

The 18-year-old has been granted strict conditional bail and is due to face Liverpool Local Court on May 26.

Reminder for parents to notify police before parties

Acting Inspector Kubecka reminded parents to notify police ahead of time if they were planning an event.

"It simply alerts police that there is a party," Acting Inspector Kubecka said.

"It certainly gives police more confidence that the parents are responsible enough ... and are willing to supervise the party.

"If police know that there is a party and whether alcohol's being served, police are at least forearmed with a bit more information before they're arrived, which may help then deal, and even assist the parents with supervising."

He said gatecrashing parties had unfortunately become a common practice in society.

"That's something that parents cannot control, but if local police are forearmed with information, they may be able to assist," Acting Inspector Kubecka said.

He said police could also inform parents of some preventative measures when planning the party, such as employing a security guard.

"Which might be deterrent enough to make sure things don't get out of hand."