A party at a Napier house went viral on Facebook, attracting hundreds of drunken youths and ending in mass fights which left several people in hospital.
Police feared at the time they could have had a fatality on their hands.
Some of the gatecrashers who turned up had gang connections, police said.
"Any one of the injuries could have resulted in a death as has tragically happened in the past in similar circumstances," Sergeant Cam Donnison of Napier Police said.
His summation of the violent circumstances was a familiar one.
"Unfortunately this is another example of youths being grossly intoxicated, many of whom were underage, and party organisers not making adequate contingencies, and having 'open' text or Facebook invites.
"Putting it out on Facebook - well that was clever," nearby neighbour to the rental Hornsey Rd, Bluff Hill, property Richard Taylor said.
The house, at the foot of the road, is about 70m from the nearest ones, and Mr Taylor was unaware at first what was happening.
"I was aware they were having a party and could hear it - pretty much every weekend they have a leer-up there."
It was not until he looked out his window and saw four or five police cars, a "paddy wagon" and a dog unit parked along Breakwater Rd that he realised all was not well. "Police blocked all the traffic from the road and I could hear them yelling out to people to get back."
He said the Friday night incident was the second time police had been called to the house in recent weeks.
For landlord Chris Jamieson the first he heard about it was after being contacted by Hawke's Bay Today.
"I'm down in Timaru at the moment and wasn't aware of it - it's disappointing to hear and I don't want tenants behaving like that. It's no good and if that's the case then they're history."
He said his cousin Ray Jamieson had been handling the rental for him and he was still trying to contact the tenants but calls had gone unanswered.
While Mr Taylor said he believed three young men rented the property, Mr Jamieson said that had been the case up until about June.
From then it had been rented to "a guy, his partner and kids".
"That's as far as we're aware but we'll be checking it out."
There was no one home at the property yesterday, except for a dog, although there was evidence there had been a party with cans, bottles and broken glass strewn about.
Mr Donnison said the eventual large force of police called to deal with about 500 mainly intoxicated youths at the property and on the street were stunned by the aggression and the attitude they faced while trying to restore order.
"They were packed in there like sardines so there was always going to be some push and shove, and you couldn't attach the animosity to gangs - it was high alcohol content in young bodies."
Alcohol, and a pack mentality, saw the first patrols to arrive just after 10.15pm pelted with bottles and abused.
One bottle smashed through the window of one of the police cars.
By that stage several people had already been taken away by their mates for medical treatment.
"At the end of the day the vast majority of these kids are reasonable kids but they get tied up in that pack mentality - the Dutch courage thing."
Mr Donnison said despite the arrival of police many of the youths continued to fight.
"In several instances they fought with each other in plain sight of police and several of the assaults involved kicks to the head while victims were on the ground."
He said police closed the party down and appealed for the crowd to disperse - but they did not.
"They refused to peaceably move on.'
After the partygoers were eventually moved on many began walking home - some a long way.
"We came across some walking towards Hastings four or five hours later - these kids had no back-up plan of what to do if things went wrong."
Mr Donnison said with the large numbers of people involved, as well as the obvious extent of "gross" intoxication and the violent atmosphere, more than 30 police were called in from throughout the district.
"That drain on resources went on for the next 2 hours and other calls from the public could not be attended because of the risks these youths posed to the general public and to each other."
When order was eventually restored, 10 youths had been arrested for charges of public disorder.
"We are still investigating several incidents - one person we understand had his jaw kicked in."
Mr Donnison said it was a timely reminder to parents to discuss party-related "issues" with their children, and have back-up plans in place as well as steer them away from drinking high alcohol RTDs.
"They have got to talk to their kids about what can happen - talk to them and eventually it will sink in."