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

New Zealand police on Friday shot and killed a "violent extremist" who was known to authorities, Prime Minister Jacinda Ardern said, after he stabbed and wounded at least six people in a supermarket.

The attacker, a Sri Lankan national who had been in New Zealand for 10 years, was inspired by the IS 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.

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 a large knife and started "running around like a lunatic" stabbing people, shopper Michelle Miller told the Stuff online news outlet.

Police Commissioner Andrew Coster told the briefing the man was acting alone and police were confident there was no further threat to the public.

"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," Coster said.

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 10 quick shots rang out.

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