New Zealand police kill "extremist" who stabbed six in supermarket

The attacker, a Sri Lankan national who had been in New Zealand for 10 years, was inspired by the Islamic State militant group and was being "monitored constantly", Ardern said.

"A violent extremist undertook a terrorist attack on innocent New Zealanders," Ardern told a briefing.

"He obviously was a supporter of ISIS ideology," she said, referring to Islamic State.

The attacker, who was not identified, had been a "person of interest" for about five years, Ardern said, adding that he had been killed within 60 seconds of beginning his attack in the city of Auckland.

Police following the man thought he had gone into the New Lynn supermarket to do some shopping but he pulled out what one witness described as a large knife and started stabbing people.

"We were doing absolutely everything possible to monitor him and indeed the fact that we were able to intervene so quickly, in roughly 60 seconds, shows just how closely we were watching him," Police Commissioner Andrew Coster told the briefing.

Coster said the attacker was acting alone and the police were confident there was no further threat to the public.

New Zealand has been on alert for attacks since a white supremacist gunman killed 51 people at two mosques in the city of Christchurch on March 15, 2019.

Ardern, asked if the Friday attack could have been revenge for the 2019 mosque shootings, said it was not clear. The man alone who was responsible for the violence, not a faith, she said.

"It was hateful, it was wrong. It was carried out by an individual, not a faith," Ardern said. "He alone carries the responsibility for these acts."

A video posted on social media showed shoppers in the supermarket seconds after the attacker struck.

"There's someone here with a knife ... he's got a knife," a woman can be heard saying. "Somebody got stabbed."

A guard asked people to leave the shopping mall shortly before about six shots rang out.

Ardern said that up until Friday, the man had not committed offences that would have led to his arrest and detention.

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

Asked if, since authorities were aware of the man, she was disappointed when she heard news of the attack, she said: "Yes, because I know we have been doing everything we could ... so I was absolutely gutted."

Of the six wounded people, three were in critical condition, one in serious condition and another in moderate condition, the St John ambulance service said.

Witnesses told reporters outside the mall they had seen several people lying on the floor with stab wounds. Other said they heard gunshots as they ran out of the supermarket.