Photo / James Stewart
Officers were pelted with cans and other objects, vehicles were overturned, fights broke out, and several fires were lit.
"It was pure luck that someone didn't lose their life," said Tairawhiti area commander Inspector Sam Aberahama.
Videos have started to emerge that show the extent of the carnage.
Beer cans and other missiles are seen being thrown between groups across two campsites.
Festival goers can be seen sheltering under air mattresses to protect themselves.
'A lot of fire and a lot of smoke'
Amelia Hooper, 19, and three friends - all from Devonport - arrived at the BW campground on December 29.
Staying with a large group of friends, the first two nights passed peacefully, she said.
They had returned from a day enjoying the nearby Rhythm & Vines festival when the carnage began.
About 5pm, the first tent was set alight in the 'Island' camp site, Ms Hooper said.
It triggered a chain of tent fires where "mostly young boys" would hold lighters to the tent fabric, and set them on fire, before moving to the next one. Ms Hooper estimated more than 30 tents were torched.
Many others were flattened by people jumping over them.
Groups then began charging fences between camp sites, sparking fights, and throwing full beer cans.
"I had my boyfriend bat away a can that could've split my head wide open. They were just flying everywhere.
"We decided to evacuate ourselves... we had to get out of there."
They watched the "complete chaos" from a distance.
Ms Hooper said there was a real fear someone could die in the skirmishes.
"It felt surreal, like it wasn't true," she said.
"It was all the boys really, the girls were just standing there watching... quite scared.
"The worst thing that happened was when one of the security guards on a quad bike got tipped and we could smell the petrol coming out... there was a lot of fire and a lot of smoke."
Police were "over-powered" by the sheer number of rioters, she said.
Once police finally gained control, they had to sleep in a car.
In the morning, once her phone had been charged, Ms Hooper said she received a "panicked text" from her mother.
She was also to assure her she was okay.
But her and her friends have "made a pact" not to ever return to BW campground.
"We definitely won't forget this New Year's in a hurry."
'A mob mentality'
BW campground director Toby Burrows said a mob mentality took over when a minority of festival goers began to cause trouble.
"It's hard to say where it starts really but they started to cause trouble, started to light fires and just create general unrest," Mr Burrows told Newstalk ZB. "That built into a bit of a mob mentality and then they start to move in mass I guess, start to do things like charge the fences and break down the internal fences and things like that."
Mr Burrows said measures in place to control behavior over the five-day event did not work on New Year's Eve.
"We put serious amount of planning and resourcing into this, especially into controlling the behavior on the 31st. There were a number of things we did all week to help control the behavior and they worked very well for the week but on the last day it didn't seem to work. It's pretty early to work out where it went wrong.
"It is very gutting. There is a lot of work and planning goes into it and it is gutting because a minority of people ruin it for everyone else. A lot of kids were very well behaved and were just there to have a good time."
Mr Burrows indicated the event's future was now in doubt if organisers couldn't work out a way to control behavior.
"There is definitely a risk that it won't continue. After any event there is always a risk that it won't continue. Hopefully it will. We're into our 12th year and there has been a lot of hard work and planning over the years. The aim is to continue it but we have to sit down and talk with the emergency services and different parties and come up with the right solutions to make it a safe event and make it feasible."
Photo / James Stewart
Today, local authorities said the riot had hurt Gisborne's reputation, and expressed doubt over whether the organisers would at least be again allowed to run a BYO arrangement.
Gisborne Mayor Meng Foon also pointed the finger at the Gisborne District Licensing Committee, and had today spoken to its chairwoman and local councillor Pat Seymour about the debacle.
"Really, what needs to be done is the local licensing committee needs to take advice from the police when the police are making submissions to the application."
Mrs Seymour said authorities had worked with the organisers toward special liquor license which would have prevented people from bringing their own alcohol.
The measure would have meant festivalgoers could only purchase alcohol on the premises, restricted the amount brought on to the grounds, and what was served to each person.
But despite this special license being granted, the organisers chose to stick with the traditional BYO arrangement after meeting "substantial feedback" from festivalgoers, she said.
After last night's events, she believed no one would want to see the BYO arrangement to continue.
She said organisers would be "very aware" that police would especially now be coming down hard on alcohol plans and oppose any other arrangement but an on-site license.
"A lot of work goes on to prevent this kind of thing so it was really disappointing to think that it happened."
When she visited the festival the evening before, she observed good behaviour from the punters and plenty of supervision and control from police and security staff.
Many were youngsters aged around 18 or 19, on their first outings away from home.
"It's extremely disappointing to think that a few people - and it would have been a relatively small number - took advantage of the situation."
'Frustrated' campers
One eyewitness took to online social forum Reddit to hit out at camp staff and police for their role in the escalating drama.
The poster, Heavy-Metal-Viking said small fires started getting lit at around 4pm and were dealt with by staff.
Around the same time, campsite portaloos were locked and barred by staff. "This just frustrated campers, and plenty of guys just dropped trousers wherever they felt like it," the poster said.
At 6pm, a mob rushed the "prison style" fences and pushed it over.
Groups then started hurling full cans at each other.
"Another contributing factor is that campers where unofficially advised to pack
everything away and take down gazebos and tents, because of people jumping into them to break them," the poster said.
"So the entire site is visible from any point because of the lack of obstructions. Any small crowd attracts bored angry teens."
Police started to "funnel" the campers toward the entrance at around 7pm, the poster claimed.
But the move created a "chokepoint" and a "big mob".
The poster said only one Rhino vehicle was tipped on its side, and not any other vehicles.
Fires consisting of air mattresses and tents were then made.
"Police where totally overwhelmed by the volume and scale," said the eyewitness.
Police said the riot was well-planned and coordinated by a core group of festival-goers who were "intent on causing trouble".
Photo / James Stewart
There was an "extremely high" level of intoxication, said Mr Aberahama.
He said there had been trouble at previous festivals, but last night's riot had been the worst.
"This was an intentional riot that was fuelled by alcohol. It was obvious a plan had been put in place by the main agitators, which quickly spread through the campgrounds. We used every available police resource to try to keep control of the crowds," Mr Aberahama said.
Mr Aberahama said police were extremely concerned about the level of disorder.
"A definite mob mentality quickly developed amongst the crowd. It was dangerous for everyone - festival-goers, police, ambulance staff and security staff."
BW Summer Festival is touted as New Zealand's "premier beachside camping festival". It runs alongside Gisborne's Rhythm and Vines, and hosts up to 15,000 campers for five days, featuring two nights of concerts at the Soundshell outdoor theatre.
About 7000 people attended the festival, in which campers could bring unlimited amounts of alcohol into the campsites.
Police said they had been unsuccessful in their attempts to oppose the festival's BYO licence.
The nature and seriousness of the injuries was not specified early today.
St John Regional Manager Stephen Smith said most of the injuries were from projectiles being thrown. People were also injured during crowd surges.
"There were a number of people who weren't interested in causing trouble and just wanted to have a good time without incident. It's a shame they had their night ruined."
Police said those arrested would face various charges ranging from disorderly behaviour to assault.
Mr Aberahama said police resources and festival security staff were stretched to the limit and extra staff had to be called in from the Rhythm and Vines festival.
Photo / NZ Police
Yesterday the BW Festival organisers posted a photograph on Facebook of "the oven" - their facility to house troublemakers onsite.
"This is where you may end up spending your New Year's Eve at BW Summer Festival if you don't play by the rules," they said.
"It has the most up-to-date shiny chrome trendy interior decor and is compact enough to hold 20 guests at a time. It also comes with high-level security grills so you don't feel unsafe.
"There's not much air-conditioning so you will be all snugly and warm. It's all onsite so you don't even have to leave your camping ground. And did we mention there are four of these lovely little chalets, all waiting for guests. We're taking bookings now for tonight, so get in quick before the rush. "
- additional reporting NZ Herald staff