Another “lone wolf” from the “religion of peace”.

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", says Prime Minister Jacinda Ardern.

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.