He said police hoped to speak to up to 80 witnesses in relation to the incident and appealed to anyone who had footage of the fight.
"Don't put it in Facebook, bring it to us."
He said the incident was serious, and it was "by the grace of God" that no one was seriously harmed.
He confirmed that police were called out earlier to the area, where a large group was gathering, but were unable to disperse the school children.
"It's difficult for us to disperse a large group of students on their way home from school."
He said at the time the students were going about their lawful business and the attending officers left the scene.
It was early days in the investigation, so Hewett was unable to say whether the brawl was in any way related to high school gangs.
It is understood that a confrontation between boys from a church incident quickly escalated when knives were pulled.
One student ran into the KFC to take cover and was chased in by the rival school group.
It is understood one student attempted to "shank" another, but thick layers of clothing protected him from being injured.
Police received multiple calls about the incident about 5pm yesterday. Four people were arrested.
A 17-year-old boy has been charged with disorderly behaviour, possession of a knife and possession of an offensive weapon.
Another 17-year-old boy has been charged with disorderly behaviour and resisting arrest. Two 15-year-old boys have been referred to youth aid.
Hewett said the matter was being investigated and it was possible further people would be charged.
"This was appalling behaviour, especially given the presence of young children and elderly nearby," he said.
"They were just trying to go about their business and this was a frightening experience for them.
"We want to assure members of the Mangere community that we will be increasing patrols in the area, including around local schools."
Police will review CCTV footage and visit schools after the students used knives, chairs and pieces of wood in a mass brawl that spilled from an Auckland street into a KFC restaurant.
Terrified staff locked themselves inside businesses on Massey Rd, Mangere, yesterday afternoon while others watched in horror as students suffered injuries bad enough to draw blood. Many filmed the violence on mobile phones.
It is understood that students from a number of schools, and some non-students, were involved in the fight. However, the Herald is unable to name the schools as those involved may be put before the Youth Court where those details are automatically suppressed.
A Mangere East KFC employee, who did not wish to be named, said about 30 students from what she believed to be two schools burst into the restaurant.
"It was a really big fight between school students. They came down to our store and started punching a guy.
"They just started smashing him.
"They just came in. They picked up chairs and everything."
She said the students were boys and girls, and looked to be under 16. Some appeared to be injured, and were bleeding and covering their faces with their hands.
A large police presence quickly swarmed the area, she said.
KFC management declined to comment last night.
"There were about 100 people there. Weapons used included knives, chairs and bits of wood," a police spokesman said.
He said CCTV would be reviewed, and "a number of" schools would be visited today.
It was not clear what the fight was over, he said.
Staff at Mangere East Library said they saw about 100 people gathered outside the KFC before the fighting broke out.
"There was a swarm of at least 50, it could've been 100 of them, and they were running up and down the road and then it all started," a staff member said.
The staff members, who wished to remain anonymous, said they could see some of the group had weapons.
They locked the doors of the library once they saw the commotion in order to protect customers.
"It was crazy. They arrested about 15 people. Some of them didn't look like they were even school kids."
Mangere East Community Centre director Roger Fowler said the fight was ugly and unfortunate, with a large group of young people watching on.
He said it was not as bad as many witnesses had described.
One school principal said he had not been contacted about the fight as of last night.
"We will be on to it first thing in the morning and will take it seriously," he said.
Appropriate disciplinary action would be taken if required, he said.
A spokesman from another school confirmed he had been informed about the fight.
"I can't comment at this stage because we are still unaware of the facts," he said. "We are going to ... get to the bottom of it."
Restaurant Brands boss Russel Creedy said CCTV footage from the Mangere store did not show much of the brawl, but said the company would be happy to provide the video to police if they asked for it.
"Unfortunately there's only one camera in that store, you can't see anything on the camera of the actual scuffle that happened inside.
"You see a few kids walking in and then they go behind an internal wall inside the store, and you can't see what goes [on] on camera.
"After about, I don't know, 30 seconds or so they walk out and they carry on [in the street]."
Creedy said staff on shift at the time called police.
The company had followed up with those working at the time, he said, and they did not seem too upset by what they had seen.
"I'm sure they were upset having seen a bit of a brawl outside.
"It's not very pleasant seeing a bunch of school kids having a go at each other so that's upsetting in itself."
He was thankful no staff or customers were hurt.
"At least that's something that's positive, that nobody seems to have been injured."
Creedy said he hoped the cause for the fight would be able to be resolved, saying school bullying was a big concern, "whether it be physical or social media bullying".
"Hopefully the school can get to the bottom of it and see why these kids are having altercations with each other."
Anyone with footage of the incident is asked to contact Detective Sergeant Steve Nightingale on 09 261 1300, or Crimestoppers anonymously on 0800 555 111.
Manukau Urban Maori Authority chief executive Willie Jackson said he knew the area where the brawl happened well - and this type of thing was not a common occurrence.
"These sort of things can happen in any area - it's not absolutely unique to South Auckland."
He said locals were shocked by the brawl, and the behaviour of the fighting kids could, in part, be attributed to boredom.
Some Mangere families found getting by was tough, trying to earn and perhaps therefore not being able to watch their kids as closely as they should be, Jackson said.
"You've got some of these parents working long hours. . . and they don't have the time to watch the kids."
But Jackson said he'd be surprised if something like this happened again in the area.
Mangere Budgeting Services head Darryl Evans agreed that the incident was a one off.
"I've worked in Mangere close to 20 years and I've never felt unsafe," he said.
He said this kind of behaviour gave Mangere a bad name, and needed nipping in the bud early.
"Parents need to know where their kids are at all times. If you allow your kids to roam the streets, they will find mischief. There aren't a lot of positive things for kids to do [in Mangere] when they're bored."
But Evans said the incident was the kind of thing that could happen anywhere in the country.
Last year, footage of Manurewa High School girls stomping and punching each other as police tried to break the fight up went viral on social media.
A Facebook page was set up earlier in 2015 which showed videos of fights between students at Auckland high schools, and police contacted the social networking giant in a bid to have the page taken down.
In May 2015 a 15-year-old girl was arrested after an assault in Kaikohe that was filmed and posted online.
And in July of the same year, a fight involving about 60 students from St Paul's College and De La Salle College broke out at Orakei train station in East Auckland over a photo posted to Facebook insulting an opposing school's rugby jersey.
