The alleged terrorist was a Sri Lankan national who arrived in New Zealand in 2011 and became a person of national security interest from 2016, authorities said.
Ardern said his ideology was IS-inspired, but he was a "lone-wolf".
"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 was gripped by ideology that is not supported here by anyone or any community.
Ardern and police commissioner Andrew Coster said nothing gave the police any reason to believe there was any further threat or risk to the public.
"We have dealt with that person. We know from our extensive monitoring that he was acting alone," Coster said.
Under "constant monitoring and surveillance"
The prime minister said she was aware of the man prior to Friday's attack, and he had been under surveillance for some time.
"By the time he entered the supermarket, he was being watched and followed. The police were with him at the time this happened - that level of surveillance was being provided because of the concerns that we had," Ardern said.
"The detailed reasons he is known to the agencies is the subject of suppression orders made by the court. In my view, it is in the public interest to hear as much as we can, in this case, I'm seeking advice on what we can do to facilitate that sharing of information as soon as possible," Ardern said.
"Of all of the tools that we have, constant monitoring and surveillance is - outside someone being in prison - one of the strongest we have, and that is what was attached to this individual," she said.
The man had previously appeared before the courts, but Ardern said "If he had reached the threshold to be in prison, he would have been in prison."
"By law, we could not keep him in prison. So he was being monitored constantly, as a result of that."
The attack is the second terror attack in recent New Zealand history. On 15 March 2019, a white supremacist terrorist killed 51 worshippers at a mosque in Christchurch.
Comment: Notably, the Christchurch attacker was recently assigned terrorist status which means the information surrounding the massacre are classified. It would appear that there are details about what happened that day that the government do not want leaked to the public: Christchurch Terror Attack: Mass Censorship, Mystery Shooters, And The Globetrotting Lone Gunman
Asked about fears of backlash against New Zealand's Muslim community, Ardern said if that occurred, "It would be absolutely wrong. The community here has been nothing but helpful and supportive, it would be wrong to direct any frustration at anyone beyond this individual. That is who's culpable, that is who was responsible, no one else."
Six hospitalised
Six people were reportedly taken to hospitals around Auckland, three of whom are in critical condition. At least 10 police cars as well as ambulances were at the scene.
Coster said the man had travelled from his home in Glen Eden, to the New Lynn supermarket, as he had done before.
"He was closely watched by surveillance teams and a technical team to monitor his actions and behaviour. He entered the store, as he has done before. He obtained a knife from within the store. Surveillance teams were as close as they possibly could be to monitor his activity. When the commotion started two police tactical operators from the STG moved to his location and engaged him. When he approached them with the knife, he was shot and killed."
"The reality is that when you are surveilling someone on a 24/7 basis, it is not possible to be immediately next to them at all times, but staff intervened as quickly as they could, and they prevented further injury, in what was a terrifying situation," he said.
The police acted within 60 seconds of the attack taking place, Coster said.
Panic and confusion
Footage shared on social media show people screaming and fleeing the Countdown LynnMall supermarket.
A witness called Tim, told Stuff he saw an elderly man lying on the ground with a stab wound to his abdomen. Another victim, a middle-aged woman, was stabbed in the shoulder, he said.
He said: "[People were] running out, hysterically, just screaming, yelling, scared." "The police came just before I was going to try and find the fella."
Also speaking to Stuff, Michelle Miller said she was doing her afternoon shop when she saw the attack unfold.
The offender was "running around like a lunatic", she said. "He was attacking people. All I heard was a lot of screaming."
She fled the supermarket as police rushed in.
Miller said she heard police ordering the man to give himself up and get on the ground, and said there was a considerable delay before she heard shots fired.
Another witness, Brittany Denyer was about to park when police officers ran up to her and told her to "hurry up and evacuate immediately".
She said: "People were panicking, and it was a gridlock trying to get out of New Lynn. Police and ambulances [were] everywhere. I was told by another police officer while I was waiting to be diverted that someone had been stabbed.
An Avondale resident Michelle Miller told RNZ: "It's horrific. I feel sorry for the Countdown staff, I feel sorry for the people who were actually hurt here today. My prayers go out to them. The police have done an amazing job. They had the guy try to give himself up but he wouldn't. It's really sad that they had to do what they had to do."
Comment: The attacker was shot dead. In other, similarly threatening situations, also where knives as weapons are involved, even ordinary police are trained to shoot to immobilize or use tasers. Bear in mind those who appparently shot the attacker were Special Tactics Group officers who were actively surveilling the man prior to the attack and they're supposed to be highly trained in operations 'beyond the scope of the ordinary police', and to be accustomed to extremely high pressure situations.
Miller said she was "pretty shaken up" and that she saw him "running around with a knife, then he just lost the plot". She did not see anyone injured first hand: "all I heard was a lot of screaming". Onlookers were "absolutely beside themselves. They can't believe that this could be so close to home. I haven't seen anything like this happen before."
Aanya Diveka was among a group of people who barricaded themselves inside the Lynmall pharmacy after gun shots rang out, RNZ reported.
She had been to get her Covid vaccination and heard screaming.
She said people were panicking and upset. One woman was struggling to breathe. There was no lock on the door so two strong men stood against the door to block it. Eventually, they were escorted out by pharmacy staff out a back exit from the mall.
Countdown Supermarkets said it was devastated by the violent attack.
The general manager of Safety, Kiri Hannifin, said their hearts were heavy, knowing what the team and customers had witnessed.
"Today's event is particularly distressing, coming so soon after the incident at a store in Dunedin Central, where four people were stabbed in May", Hannifin said.
Fatal police shootings are relatively rare in New Zealand - averaging at about 3.5 a year, according to a database of the decade from 2007 to 2017. Gun crime is uncommon in New Zealand, especially compared to other parts of the world. But it has been on the rise, despite the government's clampdown on firearms in the wake of the 2019 Christchurch mosque attacks.
Another successful deployment of 5-Eyes's Blue-Cube (or Black Cube?) run corporo-fascist "nudging"? It's more like constant harassment and socio-economic degradation, to drive "lone-wolf" attacks by means of technocratic and intimidation ploys. "Attacks" then give credence to the existence of the bloated security apparatus that are running these schemes, which they seemingly let happen though surveilling.