Robertson said it was crucial legal process was followed and the Government didn't get it wrong.
On Friday afternoon 32-year-old Ahamed Aathil Mohamed Samsudeen was shot dead by police from the Special Tactics Group after the terrifying incident where five people were stabbed and two others injured.
The man - described by the Prime Minister soon after the attack as a terrorist - was an identified threat, a dangerous high risk to the public, on a terror watch-list, and was under 24/7 surveillance.
Samsudeen initially could not be named because of heavy suppression orders around his criminal offending and immigration status.
Last night by order of the High Court, that suppression lapsed.
Today, Robertson further defended the Government's campaign to get the terrorist removed from the country.
He said law changes were imminent but no decisions had been made yet.
"It is a very challenging area," he said.
"We've got to make sure we're balancing the very important need for New Zealanders' safety here, along with a robust legal process.
"We are looking at the full sweep of the Immigration Act.
"The Government at every turn sought a remedy for this and at every turn we found we weren't able to [deport him].
"This is why we now review the overall legal framework to make sure we put ourselves in the strongest possible position to make sure this never happens again."
Robertson said he was not aware of any contact between the Government and Samsudeen's family since his death.
He said the courts had reached out through the terrorist's lawyer regarding the suppression issue, and it was likely police would have contact with them in future as their investigation forged ahead.
Three of the victims from Friday's attack remain in hospital in a critical condition after receiving injuries mainly to their torsos and necks.
Surgeons removed a 2mm knife tip from a bone of one of the injured.
A fourth patient remained in a stable condition, and a fifth had been discharged from hospital and was recovering at home.
A sixth suffered a shoulder dislocation and a seventh left the scene and went home where he treated his own minor wound.
After the terrorist's name was released, his family issued a statement.
"We wish to begin by saying that our family would like to send our love and support to those who were hurt in the horrible act yesterday," his brother Aroos said on behalf of the wider family.
"We are so shaken by what has happened and we do not know what to do.
"We hope to find out with you all, what happened in Aathil's case and what we all could have done to prevent this.
"We are heartbroken by this terrible event.
Aroos explained how the family tried to get him to change his ways.
"We realise that it will take us some time to come to terms with this. We are thinking of the injured, both mentally and physically. May we all heal from this together."
Last night the Herald revealed that Samsudeen was born in Sri Lanka who came to New Zealand in October 2011 and was granted refugee status two years later.
Immigration officials had sought to revoke his refugee status in 2018, but he appealed and a final decision had yet to be made on whether he could be deported.
His uncertain immigration status was also the reason why the terrorist could not be identified until 11pm Saturday night, when it was lifted by a High Court judge, as anyone claiming refugee status cannot be identified by law.
Samsudeen was Tamil - a minority ethnic group persecuted by Sri Lankan authorities during a decades-long conflict - and claimed he and his father were attacked, kidnapped and tortured because of their political background.
His claim to asylum was supported by scars on his body, as well as a psychologist's report which said Samsudeen presented as a "highly distressed and damaged young man" suffering from post-traumatic stress disorder and depression.
WHERE TO GET HELP:
If you are worried about your or someone else's mental health, the best place to get help is your GP or local mental health provider. However, if you or someone else is in danger or endangering others, call police immediately on 111.
OR IF YOU NEED TO TALK TO SOMEONE ELSE:
• LIFELINE: 0800 543 354 or 09 5222 999 within Auckland (available 24/7) • SUICIDE CRISIS HELPLINE: 0508 828 865 (0508 TAUTOKO) (available 24/7) • YOUTHLINE: 0800 376 633 ,free text 234 or email talk@youthline.co.nz or o nline chat. • NEED TO TALK? Free call or text 1737 (available 24/7) • KIDSLINE: 0800 543 754 (available 24/7) • WHATSUP: 0800 942 8787 (1pm to 11pm) • DEPRESSION HELPLINE: 0800 111 757 • SAMARITANS – 0800 726 666.