An ambulance service is asking why it was unable to get urgent police 111 assistance when an ambulance was under attack by drunken youths at the weekend.

Taranaki District Health Board ambulance chief Superintendent Ian May said yesterday that he was personally crewing the ambulance at Mangorei Rd, New Plymouth, after it had picked up an injured youth about 11pm on Friday.

The youth had suffered facial burns after a sky-larking prank went wrong at the party.

Police were already at the party house, further up the driveway, when the ambulance arrived.

Police were closing down the party after complaints, including the youth being set on fire, others doing donuts in vehicles, and noise.

"Then a large party congregated around the ambulance," Mr May said.

"A large number of drunken youths started to bang on the side of the ambulance. They were unruly, using obscene language and throwing bottles through the window.

"We requested urgent police assistance."

But the ambulance crew was left stranded when the ambulance communication centre was unable to contact the police communications centre to alert the nearby police.

When there was no help in sight, ambulance staff decided to get away from the area - experiencing considerable problems backing out of the driveway without running over any of the out-of-control youths.

"It was a very scary situation for the ambulance crew," Mr May said, adding that the ambulance service had no criticism of police at the scene.

"We have an extremely good relationship with them. The 111 service let us down."

Mr May said he would investigate what had caused the communications breakdown to ensure ambulance crews' safety in future.

It was concerning that an ambulance had been the target of the youths who had acted in drunken pack behaviour.

"While we didn't fear for our safety at that point we were concerned it could turn to something more damaging or dangerous."

Inspector Gary Allcock, head of police central communication centre in Wellington, said yesterday that initial investigations had determined the centre had been very busy at the time.

The ambulance had left by the time the dispatcher had alerted police at the scene, Mr Allcock said.

There had been 213 111 calls from 10.30pm to 1.30am on Friday - many of them about the Mangorei Rd party.

"It was an exceptionally busy night. At the end of the day, when we're that busy, that's the way it is."

Ambulance 111 calls were not given priority over other emergency calls.

Taranaki Ambulance Service did not use a direct communication system used by other ambulance services and preferred the present system, Mr Allcock said.

A Government inquiry into the 111 system was begun last year after the Iraena Asher case, in which a taxi was dispatched following her call for police help.

The 111 system came under scrutiny again last month after a sexual assault victim was told to make her own way to the police station.