The alleged offender is in custody after four people were stabbed at the Countdown supermarket on Cumberland Street in Dunedin this afternoon.
Southern District commander superintendent Paul Basham says the incident appears to be a "random attack".
Just before 2.30pm today, emergency services received the first calls reporting a number of people had received stab wounds inside the supermarket, he said
According to Basham, police were at the supermarket within minutes and found four people with serious stab-related injuries.
"We also located a man, who had been detained by members of the public, that we believe is allegedly responsible for this incident," Basham said at a press conference this evening.
He says the investigation is in its early stages and the motivation for the seemingly random attack remains unknown.
The alleged offender is currently receiving medical attention in Dunedin Hospital after being injured while members of the public detained him.
"He is under police guard and is yet to be formally spoken to or charged," Basham said.
"We would encourage anyone who has information, including video footage of the incident, to get in touch with police via 105 and quote event number P046456846."
Basham also joined Prime Minister Jacinda Ardern in praising the "heroic" efforts of those who detained the man.
"I viewed traumatic CCTV footage which captured those who intervened, some who were injured themselves as they acted selflessly and with great courage to prevent further injuries," he said.
The incident happened adjacent to the pharmacy area.
Ardern said earlier that police had told her there is "nothing to suggest that this is a domestic terror event".
According to a Southern DHB spokesperson, "Five people have presented to the emergency department as a result of a stabbing at Countdown supermarket today, and are 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."
If you have any information about this incident, email firstname.lastname@example.org
A witness told the ODT she was at the checkout when someone said a person "had a knife".
The woman then said she heard people screaming.
One witness said she believed the attacker was on drugs. Another told the ODT the man was having a psychotic episode and used two knives to stab people.
Two of the victims were Countdown employees, Countdown spokesperson Kiri Hannifin said.
Countdown released a statement on the incident on their Facebook page which reads, "We are shocked and devastated by the events in our Dunedin Central Store this afternoon.
"Our priority right now is our injured team members and caring for our wider team in the wake of this extremely traumatic event. We are deeply upset that customers who tried to help our team members were also injured.
"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.
"We have been concerned about the escalating violence towards our team, and this is something we have continued to talk and raise as an issue over the last year.
"The store will remain closed today and tomorrow and we are co-operating fully with the police."