Jenny McDowell was just metres away from the horror unfolding at Countdown when a man who had just stabbed a woman stopped and looked at her.

Fearing she might become his next victim the terrified woman kept her eyes diverted.

"There was a brief second where the man who attacked, he briefly looked in my direction, and I thought he was going to come for me next," McDowell told the Herald.

"There was just anger in his eyes. He kind of looked in my direction, but I didn't make direct eye contact with him."

The two Countdown employees injured in the attack were recovering this morning in hospital.

"We were very worried about them overnight but they are doing ok," Countdown head of corporate affairs Kiri Hannifin told the AM Show.

Police said a 42-year-old man had been charged with four counts of attempted murder.

After treatment at Dunedin Hospital he had been moved to the central police station ahead of an appearance at the Dunedin District Court today.

Three of the injured remained in a serious but stable condition, while a fourth person injured in the incident was in a moderate condition.

McDowell was in the shampoo and conditioner aisle at Countdown on Dunedin's Cumberland St when she heard screaming just after 2.35pm.

"I looked up and witnessed a man stabbing one of the female staff members," she said

"I just felt fear and panic but I just slowly backed away a little bit."

As the drama continued to unfold staff and members of the public tried to restrain the man using whatever methods they could, including throwing things at him.

"They would've been about three or four people, he almost got loose again. One man had a chair on him with his pressure on it. It all happened so quickly.

"While they were trying to restrain him, before they got him restrained, they were trying to throw things at him and to distract him."

Police arrived soon after and took the man, who was treated in hospital for injuries, into custody.

By the time he left the store in handcuffs, four people had been injured - three of whom critical and last night undergoing surgery. The fourth was in moderate condition.

Some of the injured were members of the public who restrained the man and have been praised by police for their actions.

"The credit goes to the members of the public who acted heroically [to try and stop the alleged offender]," said Southern police district commander Paul Basham, who described footage of the attack as "traumatic".

"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 and indeed I think they did just that."

McDowell was among those who tried to help one of the injured, a man with a wound to his neck that was bleeding heavily and cuts to his thigh and lower back.

"Someone yelled at me 'can you administer first aid?' I froze for a second or two. I had trained in first aid years ago. I thought, 'I have to get something to stop the bleeding'. I had a reusable bag so I emptied that out."

Before she could hold the bag over the wound, a Countdown staff member had taken his jumper off to use instead.

"I just tried to help apply pressure but it was mainly [the worker] he was amazing. I was just helping to hold down the jersey to get the pressure on. The man who was injured [was saying] – 'my wife, is my wife OK?'

"I just tried to reassure him that help was on the way and stayed with him. "

Witnesses tell of 'chaos' in aisles

Other witnesses in the supermarket said there was "chaos" as the attack unfolded.

A couple, who asked not to be named, were shopping in the aisle next to the pharmacy department.

"We just heard a scream and thought someone must've fallen, but it got louder and louder and more people were screaming," the woman said.

"I saw one red hand among the people; a bloody hand, and everybody started running towards the exits."

Another woman was also alerted to the incident by the high-pitched yelling.

"I heard screaming and I thought it was just teenagers, but then I heard someone yell 'he's got a knife'."

After the attack staff members congregated outside the supermarket, some visibly shaking.

Arthur Taylor, who had just walked into the supermarket car park when a police car came flashing past him, said a Countdown worker emerged "in a hell of a state".

He sat with him and tried to calm him down.

"He must've witnessed it all. He was sweating, couldn't string many words together at all," said Taylor.

Countdown managing director Spencer Sonn said they were shocked after the attack.

"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.

"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.

"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."

The store will remain shut today as the police investigation continues.

Hannifin described the actions of shoppers as "amazing and humbling".

"You don't come shopping in our business to save lives but that's what happened yesterday and you don't come to work to get stabbed and that also happened yesterday."

Hannifin said there had been an increase in abuse and assaults in its stores across the country. Countdown New Zealand would now look at whether it should roll out bodycams in its stores as it already had in Australia.

"Our team will all be feeling this because safety is such a focus for our business and keeping safe - we talk about it every single day so I speak for all of our team when I express our devastation."

Basham said part of the investigation would include looking into the motivation for the attack.

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

Prime Minister Jacinda Ardern said there was nothing to suggest a domestic terror event.

"Needless to say such an attack is hugely concerning."

Ardern acknowledged courageous efforts by bystanders and said her thoughts were with all those injured.

Basham said the man would be formally charged last night or first thing today and appear in the Dunedin District Court this morning.

He said the man would face at a minimum, wounding with intent to cause grievous bodily harm - but that could change.

"God forbid the status of those victims deteriorates overnight, and then we would be looking at something else. But let's hope that's not the case.

- Additional reporting: Otago Daily Times