A witness has described the harrowing moment a man began a frenzied, Islamic State-inspired knife attack in west Auckland.

Six people were injured at New Lynn Countdown on Friday afternoon, including three critically, after a Sri Lankan national apparently took a knife off one of the shelves and began stabbing people, before being shot dead by police just a minute later. 

Shaneena Balasundram was just metres away from the attack, after coming face-to-face with the 32-year-old terrorist.

"I went to Countdown and I was going through the dairy/milk aisle and this guy out of nowhere just started stabbing this lady out of nowhere, who was also looking in the dairy/milk aisle."

She said she felt helpless to do anything - not even flee. 

"I'm so sorry I couldn't do much - I just froze. I wish I could do more… He stabbed her again, 'Allahu akbar, allahu akbar'... when he looked at me, he didn't seem to be interested in me." 

Warning - video contains footage that may upset some viewers.

The man had been under police surveillance since 2016, but for legal reasons couldn't get him behind bars. Within a minute of the attack starting, it was all over - armed police shooting him dead.

Balasundram says she is traumatised, and still coming to terms with what she saw.

"What the hell? Why? … Why did you do this? It's just not fair, it's not right. Come on, what sort of world are we living in here?"

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St John said on Friday night three patients taken to Auckland City Hospital were in a critical condition and one serious; one patient in a moderate condition was taken to Waitakere Hospital; and one in a moderate condition was taken to Middlemore Hospital.

The attacker had been on the state's radar since 2016, having been caught with Islamic State propaganda. 

He was allegedly arrested after planning a "lone wolf" knife attack in Auckland earlier this year, but couldn't be charged as a terrorist because - thanks to a loophole in the law - preparing a terrorist attack is not in itself deemed an offence under New Zealand's counter-terrorism legislation, if no action is taken towards carrying it out. 

He was instead prosecuted on lesser charges, the Herald reports, and avoided prison. 

Waitakere Councillor Shane Henderson said locals were stunned and shocked, not expecting something like this at all to happen in New Lynn.

"You sort of wonder what would happen if your family was there," he told Newshub. "We as Westies won't forget our values of showing kindness and friendship to each other… This person is obviously filled with hatred and division, that's not west Auckland's values."

He said the police did a "fantastic job" to stop the attack inside of a minute, but wants legislators to take a close look at the law.