Five people - including the attacker - were taken to Dunedin Hospital on Monday afternoon after the shock attack at a supermarket in the South Island city.

St John says three people had critical injuries and another two were moderately injured.

Ambulances and armed police attended the scene within minutes of being notified, at about 2.30pm (NZST) on Monday.

Superintendent Paul Basham said that before police arrived on the scene, bystanders apprehended the man despite risks to their own safety.

"It's nothing short of heroic," he said after reviewing CCTV footage.

"Those that intervened, some of whom became injured themselves, acted selflessly and with great courage."

The attack took place at the Countdown Dunedin Central, just metres from the iconic Dunedin Railway Station and the city's heart, the Octagon.

Witnesses told news website Stuff the attacker was "quite a messy little fellow" with curly hair.

The alleged offender is under police guard at Dunedin Hospital, and is yet to be formally interviewed or charged, police say.

"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," Mr Basham said.

"On the face of what we currently know, we believe this was a random attack."

Ms Ardern called the attack "hugely concerning", and said police had advised her there was "nothing at this stage to suggest it was domestic terror".

"I do want to acknowledge really early reports of courageous acts by bystanders who have taken action in order to proetect those around them," she said.

"Our thoughts are with all those who've been affected by this attack."

Countdown spokeswoman Kiri Hannafin said two of the injured were employees.

A statement from the company said they were "shocked and devastated", and the store would be closed until Wednesday.

"We are deeply upset that customers who tried to help our team members were also injured," it read.

"Our priority right now is our injured team members and caring for our wider team in the wake of this extremely traumatic event."

Australian Associated Press