Prime Minister Jacinda Ardern said it would have been "very, very difficult" to deport Ahamed Aathil Mohamed Samsudeen, the New Lynn mall terrorist.
However, she said the Government will explore whether there were other avenues that could have been taken to deport him.
Ardern has come under fire from Opposition Leader Judith Collins, who argued that two sections of the Immigration Act would have given the Government the grounds to deport Samsudeen.
"Immigration Law in New Zealand provides that the Minister of Immigration can certify that a person constitutes a threat or risk to security and, with Cabinet's approval, the Governor-General can order that person be deported," Collins said.
Collins noted that the Refugee Convention allowed countries to deport refugees on the "grounds of national security or public order", which would likely cover Samsudeen.
But Ardern noted that the threat of persecution in Sri Lanka would have made it very difficult to deport Samsudeen.
She said there were two roadblocks to deportation. He was currently in the midst of a tribunal process, and Crown Law advised that he would likely be a "protected person" as he would face persecution if he was deported to Sri Lanka.
"He was going through a tribunal process... and had appealed the cancellation of his refugee status," Ardern said.
"One of the issues that would be considered by the tribunal, however, is whether or not the individual was a protected person.
"If someone was a protected person that makes deportation very, very difficult," Ardern said.
But Ardern said she would continue to press for further information on whether there was ever an option to deport the man, saying "there are a few things we can explore".
"The question I have specifically was when someone has fraudulently obtained that [refugee] status then poses a national security threat - there is a bit of work to be gone through there," Ardern said.
"Most New Zealanders will say if you acted fraudulently in gaining that [refugee] status and you're a threat to national security, surely there are some mechanisms that we can use there - so that's what we're exploring," she said.
Ardern repeated her promise that changes to counter-terrorism laws would be passed by the end of the month - something Collins has offered to support.
Act and the Greens have both expressed their concern at moving forward so quickly.
The Greens' Golriz Ghahraman said the attack "must not be used to rush counter-terror laws without scrutiny.
"Our response must capture the complexity of radicalisation, protect rights and guard against causing more harm to the most marginalised communities."
Act leader David Seymour said the bill the Government intends to pass should be split into multiple bills.
One would make "simple and immediate changes, if applicable," while the other should be given additional time "to consider new information in light of the Lynn Mall attack."