The man who stabbed six innocent supermarket shoppers today in a terrorist attack - before being shot dead by police - was an "Isis-inspired known threat" who was under "constant police surveillance", Prime Minister Jacinda Ardern says.

The man was a known threat risk since 2016 and had been previously arrested for allegedly planning a "lone wolf" knife attack.

He cannot currently be identified because of court suppression orders - but the work is already under way tonight to unmask the man.

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"The Crown is seeking the courts' urgent lifting of the suppression orders, which will be filed this evening," a spokesperson for the Prime Minister said tonight.

Police were following the man after he left home and walked into the New Lynn Countdown this afternoon - before he took a knife from a shelf, and carried out the attack.

Three of the six shoppers are in a critical condition in hospital.

"This was a violent attack on innocent New Zealanders, it was senseless," said Ardern. "It was undertaken by an individual who was a known threat."

The man was under watch because of his extremist views and sympathies with Isis.

"The terrorist is a Sri Lankan national who arrived in 2011," Ardern said.

Police Commissioner Andrew Coster said the man was "closely watched by surveillance teams and a strategic tactical team" as he travelled from his home in Glen Eden to Countdown in New Lynn this afternoon.

He took a knife from a shelf in the supermarket before carrying out the attack. He was shot and killed by police within 60 seconds.

The reality is when you are surveilling someone on a 24 hour basis it is not possible to be immediately next to them," Coster said.

"I want to acknowledge the six New Zealanders injured," Ardern said. Three were in a serious condition.

"The terrorist is a Sri Lankan national who arrived in 2011," said Ardern

Ardern said there was information about him is covered by court suppression orders, she said.

But the Herald has previously reported the case of the man - a known Isis sympathiser whom the police had previously arrested for allegedly planning a "lone wolf" knife attack.

"We have utilised every legal and surveillance power available to us to keep people safe from this individual," Ardern said, adding the man became a person of interest in 2016.

There would be multiple inquiries following today's terror attack, Ardern said, those by the IPCA and coroner.

"What happened today was despicable, it was hateful it was wrong. It was carried out by an individual, not a faith, not a culture, not an ethnicity, but an individual person who is gripped by an ideology that is not supported here by anyone."

Ardern said she intended to wait for the reports to release any information she could.

"He alone carries the responsibility for these acts. Let that be where the judgment falls," Ardern said.

The man was fatally shot by police officers while three of the six shoppers are understood to have suffered life-threatening injuries in the horrific scenes.

The three critical patients, and another in a serious condition, have been taken to Auckland City Hospital. One person is in a moderate condition is at Waitākere Hospital, and another is in a moderate condition at Middlemore Hospital.

"As the commissioner has said there was nothing to indicate that what he did today was going to happen today," Ardern said.

Ardern said she had been advised that she could not share the timeline of the man's interaction with the state - including whether he had recently been released from prison.

Asked why six people were able to be injured before he was shot - when he was under surveillance - Coster said the man was very "surveillance conscious" and police had to keep some distance from him to make surveillance effective.

A brave bystander at the mall helped a wounded woman who had been stabbed in her right hip.

Asked again why somebody who was deemed a threat to national security was allowed in the community, Coster said police had done as much as they were able to under the law.

Ardern said the man held a 'violent, Isis-inspired' ideology but said she did not expect a backlash in the community.

"That would be wrong, that would be absolutely wrong - the community here has been nothing but supportive," Ardern.

Ardern was not sure when the suppression orders would be able to get lifted.

Ardern said it was "speculative" that today's actions could have been prevented by anti-terror law changes that are currently before Parliament but not yet passed.

Ardern was confident he was a lone wolf.

As people scrambled to evacuate others yelled "he's got a gun" and the injured woman got up and fled the scene.

A witness told the Herald their adrenaline was still pumping after the horrific attack.

Auckland City Hospital has multiple operating theatres running to treat the injured - who have suffered chest and neck wounds. Two major abdominal cavity surgeries are under way, the Herald has been told.

Ardern said the terror alert level would remain at medium.

There were "very few" people in the same category as the man.

"I can tell you that agencies were using every single possible means available to them to protect the New Zealand public form this individual - every single possible means" Ardern said.

Ardern would not say whether he was a citizen, or what his immigration status was. She said she would reveal as much as she could when she was able to.

"I was absolutely gutted," Ardern said. "The reason he was in the community was because of his inability through the law to have him anywhere else."

"We used every element and lever in the law that was available, and when he was in the community there was constant monitoring," Ardern said.

Coster said the surveillance was being undertaken as a "public safety operation".

Ardern said: "If we had reached a threshold for him to be in prison, he would have been in prison."

SIS report

The 2020 annual report of the SIS said: "During the past year, between 30 and 50 individuals have been under active investigation by the NZSIS in relation to violent extremism at any one time."

The number was higher than in recent years, the report said, due to investigations into politically-motivated violent extremism, including white identity extremism.

"That said, faith-motivated violent extremism continues to appeal to a minority of individuals in New Zealand.

"Over the last year we observed individuals in New Zealand with probable intent and almost certainly the capability to engage in acts of politically-motivated violent extremism and faith-motivated violent extremism.

"Terrorist violence is most likely to come from lone actors, including those who may mobilise to violence rapidly with little or no warning. Our active investigations include a small number of New Zealanders who are believed to remain in Syria or Iraq with ISIL or other extremist groups."

Terror expert Dr John Battersby said the Government had been "pedestrian" in dealing with the Terrorism Suppression Act.

"It doesn't surprise me. We have lulled ourselves into a false sense of security that Christchurch was an abhorrent action," he told Newstalk ZB's Heather Du Plessis-Allan.

"We need to look at the powers and resources that our intelligence agencies have."

Countdown won't open until 10am Saturday

Every Countdown supermarket in New Zealand will not open until 10am tomorrow.

"We are devastated by this violent attack in our Lynn Mall store today," a spokeswoman said.

"Our whole team will be deeply affected by what has happened, and we want them to have a chance to talk and to get the support they need."

Auckland Mayor Phil Goff vents frustration

Auckland Mayor Phil Goff says it's frustrating not all the details of the attacker can be revealed because of a court order.

He knew nothing about the individual beforehand. But he said from his experience in government, the amount of people like this individual being monitored are extremely few.

"What's frustrating for all of us who have just listened to the Prime Minister and the police commissioner, is that they were constrained from sharing all of the information that they know about this individual, because of a court order, and I understand that the Prime Minister of all people have to live under the law, and cannot go outside the law.

"But what is clear about it is that this person is an ideologically motivated terrorist... It's appalling that he could cause so much hurt in the space of a minute, just with a knife. But I guess the police interceded as quickly as they were able and ultimately, he has paid the price of his actions."