The stabbing of four people this afternoon is believed to be a random attack. The alleged offender has been taken to hospital with injuries, and has not yet been charged.
Police district commander Paul Basham said they received the first of many calls about the attack just before 2.30pm.
He said there were four victims, and a man who had been detained by members of the public before police arrived.
The man had a minor injury and would appear before the courts tomorrow, Basham said. Police were satisfied they had the person responsible in custody, he said.
Countdown corporate affairs manager Kiri Hannifin earlier confirmed there had been people stabbed at the supermarket in Dunedin. She said two of them were employees.
In a statement, Southern DHB said five people had presented to the emergency department as a result of a stabbing at Countdown supermarket today and were being assessed and treated.
"Four patients are being admitted to hospital in a serious condition, three to the ICU and one to general ward. One patient is being treated in the emergency department."
Basham said three of the victims were in "status one" and were going into surgery this afternoon.
Police have closed the store down.
One witness said she believed the attacker was on drugs. Another said a man was having a psychotic episode and used two knives to stab people before being disarmed by people in the store including staff.
Basham said the alleged attacker was under guard at Dunedin Hospital, and was expected to be returned to the police station tonight or early tomorrow before appearing in court tomorrow.
Dunedin Mayor Aaron Hawkins said the incident was not something he would expect to unfold in his city.
"Everyone should feel safe in their place of work or going about everyday tasks like grocery shopping. These sorts of events are particularly chilling because it could have been any of us or our friends or family - it's a busy location in the centre of the city."
'We believe this is a random attack' - police
Basham said the investigation was in its early stages, and the scene was locked down, but the attack was believed to have been random.
"On the face of what we know we believe this is a random attack."
He said the actions of the people who detained the man were "nothing short of heroic".
"This was a fast-moving and extremely traumatic event for every person in that supermarket - the victims who were injured, those present who tried to intervene and those who had to flee to a place of safety.
"Those who intervened, some of whom became injured themselves, have acted selflessly and with great courage."
He said police were seeking to establish the identity of our victimes and to get in touch with their next of kin.
They were working to compile witness information and collecting CCTV footage, he said.
People with information, including video footage, were encouraged to contact police on 105 and quote event number P046456846.
Nothing to suggest domestic terrorism - Ardern
Prime Minister Jacinda Ardern, speaking after a Cabinet meeting, said there was nothing to suggest it was a domestic terror event.
"Needless to say, such an attack is hugely concerning and I do want to acknowledge the really early report of courageous acts by bystanders."
"Our thoughts are with all those who've been affected by this attack."
She said police were still working through the motivation of the attack.
"They want to have a bit more time before they make public statements on that," she said.
Police were not looking for any other individuals and there was no reason to believe anyone else is involved, Ardern said.
"There is nothing to suggest there is more than one person involved at this stage."
St John Ambulance said all the injured were taken to Dunedin Hospital, three in a critical condition and one in a serious condition.
Escalating violence towards supermarket staff - Countdown
Hannifin said the company was trying to suppport the two injured team members and their family and also the rest of our store.
She said there has been 13 to 14 months of increasing violence to supermarket staff, including horrendous verbal abuse.
"Horrendous verbal abuse, physical abuse. On Thursday night someone tried to burn down our store in Henderson while our team were working in it. So it's just unacceptable violence.
"Every day in we have in our stores an assault of some variety. There seems to be no rhyme or reason apart from a complete disregard and lack of respect for our people."
She said there were very few details at this stage, but it looked like there was an incident in the health and beauty area near the pharmacy.
"I'm having trouble ... to talk to the managers in the store but I'm having trouble getting much out of them because ... they're in shock, but also trying to deal with injuries of those customers and teams. I'm in quite a bit of shock myself."
Basham said police would be looking at any increased security at supermarkets.
In a statement, Countdown said it was "shocked and devastated" by the events, and its priority was the injured members of staff and caring for their team.
The supermarket chain said there were concerns about escalating violence towards staff members.
"This is something we have continued to talk annd raise as an issue over the last year.
"Our amazing team comes to work to serve Kiwis every day, and our customers visit our stores knowing that they will be safe. We are heartbroken that this wasn't the case today."
Photo: Supplied/ Arthur Taylor
Photo: Supplied/ Arthur Taylor
Photo: RNZ / Tim Brown