New Zealand should have got rid of the west Auckland terrorist, who injured seven people in a knife rampage on Friday, much sooner than they were trying to, a law expert says.

On Friday afternoon, Sri Lankan national Ahamed Aathil Mohamed Samsudeen, a man that officials had been trying to deport for years, carried out an "ISIS-inspired" attack with a knife on shoppers at the Countdown in west Auckland's LynnMall.

Questions have since been raised about why the 32-year-old was hard for agencies to deport. 

"Immigration New Zealand explored whether the Immigration Act might allow them to detain the individual while his deportation appeal was heard," Prime Minister Jacinda Ardern said on Saturday. "It was incredibly disappointing and frustrating when legal advice came back to say this wasn't an option." 

Law Professor Al Gillespie, from the University of Waikato, says he should have been sent packing as soon as suspicions arose.

"Once we do the review, we must never let this happen again," Gillespie told Newshub. "When we've got a significant threat to national security, our need for safety in the community trumps all of those other considerations."

The attacker had previously pleaded guilty to possessing ISIS material and was known to police.

Samusdeen, a Tamil Muslim who arrived in New Zealand in 2011, was granted refugee status in 2013. However, years later, he was notified this would be revoked, something Samusdeen challenged.

Gillespie said while "the debate gets difficult" considering he was a refugee, "if there's grounds of national security, even if there's a risk to him being returned [to Sri Lanka], then that is the trumping factor and that's what should have happened here".

READ MORE

• Mother of LynnMall terrorist claims neighbours 'brainwashed' him as he recovered from injury
• LynnMall terrorist's family: 'We are heartbroken'
• Attempts to strip terrorist of refugee status 'frustrating process'
• Auckland terror attack: Authorities spent years trying to deport terrorist

Meanwhile, Mental Health Foundation chief executive Shaun Robinson is dismissing claims the Mental Health Act failed the terrorist.

The terrorist's family has said Samsudeen was suffering from "some mental health problems" which had deteriorated in the past decade.

Robinson told Newshub the incident was a terrorism law failure.

"The Mental Health Act is about supporting people and their health - it is not about punishing people or keeping the community safe from criminals. The criminal system and the anti-terrorism system is what needs to do that."

Corrections has confirmed the terrorist was serving time at Auckland Prison until July. High-profile prisoner and advocate Arthur Taylor claimed the supermarket terrorist was tough for security guards to deal with.

Taylor said another inmate had confided in him about Samsudeen's "extreme" behaviour.

"All this would have been known - this [information] would have been available. This is what I'm seriously concerned about... seven people have been seriously injured here."

As of Sunday, three of the seven victims remained in a critical condition.