The Government has revealed some of the documents the LynnMall terrorist used to gain refugee status were found to be fabricated.

That's why it tried to cancel his refugee status two-and-a-half years ago, but even as he carried out his attack on Friday, the Government was still trying to get him out of the country.

"At every opportunity, we have been looking for ways to deport this individual," Deputy Prime Minister Grant Robertson told media on Sunday.

After his initial application was refused in 2011, an Immigration and Protection Tribunal Hearing granted Aathil Samsudeen refugee status in 2013 after finding "a well-founded fear of being persecuted if he were to return to Sri Lanka".

But that was cancelled in 2019, because Immigration New Zealand found his application was fraudulent.

"Further down the track a different group of people doing a different investigation obtained some material that indicated to them the likelihood this looked like it was fraudulent," Robertson explained.

Samsudeen appealed, and as the legal battle dragged on, he carried out his attack.

"We effectively had a time bomb in this country, and we knew about it for a number of years, and we couldn't get them out," says Waikato University law professor Alexander Gillespie.

ACT leader David Seymour says the fraudulent documents should have been the smoking gun.

"That was his ticket out of here. Because of the tardiness recognising that, he was still here and we've had this heinous attack."

Even though Samsudeen was a known threat to the New Zealand public, Immigration New Zealand couldn't have him detained while his deportation appeal was heard. The Immigration Act only allows someone to be detained for the purposes of deportation.

Another tribunal hearing was pending.

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 "The Government is not above the law, and we needed to go through that. And so yes, I know, on the surface when you read it, it's a very long timeframe. But that is the nature of these kinds of claims," Robertson says.

But it was unlikely Samsudeen would be deported anyway, because he would qualify as a protected person due to the treatment he'd probably receive if he returned to Sri Lanka.

"If there is a possibility that he is at risk of harm back in Sri Lanka then it's necessary to tread carefully, because New Zealand has obligations under the Refugee Convention not to return someone to a place of harm unless there are very good reasons to do so," says immigration lawyer Simon Laurent.

He says the system is designed to afford protection.

"If you're going to take that protection away then you're going to have to be pretty damn sure you're doing it for the right reasons."

Prof Gillespie says a known threat to New Zealanders is reason enough.

"When we've got a significant threat to national security, our safety in the community trumps all of those other considerations," he says.

The Government is aware of the frustration.

"At every turn the Government sought a remedy to this. And at every turn, we found that we weren't able to. That means we've got to go back and take a look," Robertson says.

He's indicated more law reform is coming.

"We are looking at the full sweep of the Immigration Act, what the powers are within that."

ACT says a protected person should be deported if the danger to themselves is less than the danger they pose to New Zealanders.

"We need to change sections 163 and 164 of the Immigration Act so a protected person is not just protected from being deported, but New Zealanders are protected from them staying here," says Seymour.

Laurent, who has experience representing refugees, is pleading caution.

"Most people who come in as refugees and are recognised as refugees have a very good reason for doing so. This is not a walk in the park for them. They are mostly, and I know quite a lot of them, people who contribute to the community and are grateful to be here."