Four people were injured, three critically, when the man began his attack at the Countdown store on New Zealand's South Island, police said.

Supermarket chain Countdown, a subsidiary of Australia's Woolworths, said two employees were among the victims.

The alleged attacker also suffered minor injuries during the incident, Southern District Police Commander Superintendent Paul Basham said.

"He is in Dunedin Hospital, we have a team down there talking to others," he said.

"I expect when he is seen at the hospital he'll come back to the police station tonight and be formally charged this evening and appear in court tomorrow morning if all goes to plan."

The three victims who are in a critical condition will undergo surgery later this afternoon, Superintendent Basham told media.

The identity of the victims has not been revealed.

he attack happened around 2:30pm local time, near the pharmacy section of the store.

Superintendent Basham said some shoppers who witnessed the attack confronted the man and prevented him from injuring more people.

"I have viewed the CCTV footage, which is quite traumatic, and what I can say is that those that intervened, some of who became injured themselves, I think have acted selflessly and with great courage to prevent this man from hurting anybody else," he said.

"It is nothing short of heroic."

Countdown issued a statement announcing the store would be closed until Wednesday.

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

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

"Dunedin Central store will remain closed today and tomorrow while we support our team."

The company also said there were concerns about escalating violence towards its employees in recent months.

"This is something we have continued to talk and raise as an issue over the last year," the statement said.

"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."

Story By | Michael Doyle ABC