The man, who was shot dead by police, arrived in Aotearoa in October 2011, lived in the west Auckland suburb of Glen Eden and was "ISIS-inspired", according to Prime Minister Jacinda Ardern.
Six people were injured in the attack at the New Lynn Countdown supermarket - three are in a critical condition and one is in a serious condition.
"This was a violent attack, it was senseless and I'm so sorry it happened," said Ardern.
"It 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.
"He alone carries the responsibility for these acts. Let that be where the judgement falls."
Ardern and Police Commissioner Andrew Coster revealed the details at a media briefing on Friday evening.
The individual was "known to multiple agencies", had been placed on a terror watchlist in 2016 and was under surveillance during the attack. Officers monitoring him were close enough to hear the attack begin to take place in the LynnMall Countdown and were able to stop it in around a minute.
"It was the police surveillance team and Special Tactics Group (STG) who are part of that monitoring and surveillance that shot and killed him within the space, I'm told, of roughly 60 seconds of the attack starting," said Ardern.
The man is said to have used a knife in the attack that he acquired from within the supermarket.
"This individual was under heavy surveillance as a consequence of concerns about his ideology," said Coster.
"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."
Coster said the officers intervened as quickly as they could "with great courage" in the terrifying situation. He said they couldn't have been following any closer to the attacker as that would have compromised their surveillance operation.
Those monitoring the extremist were given no indication he was intending to carry out the attack when he did, Coster said, noting he regularly visited the LynnMall Countdown.
Ardern and Coster insisted everything possible that could be done under the law had been to try and keep the extremist behind bars, adding suppression orders mean details can't yet be revealed.
"We have utilised every legal and surveillance power available to us to try and keep people safe from this individual," Ardern said.
"The fact that he was in the community will be an illustration of the fact that we haven't succeeded in using the law to the extent we would've liked. That is why he was being closely monitored at all times.
"By law we could not keep him in prison. If he'd committed a criminal act that would've allowed him to be in prison, that's where he would have been. Unfortunately, he didn't."
Ardern said she had personally known about the extremist for "some time". She wanted to share more information detailing every attempt made by authorities to prevent him carrying out an attack, but said she had to wait until releasing it was approved by the courts.
She sent a heartfelt thanks to the members of the public who rushed to aid the injured victims.
Ardern said there would be no change to New Zealand's terror alert level and wouldn't comment on whether she was concerned about retaliation attacks.
The extremist was described as a lone wolf and no collaborators are suspected, she said.