Foreign Affairs Minister Nanaia Mahuta was told in advance Australia would deport a 15-year-old child to New Zealand.
Speaking to reporters on Tuesday morning, Mahuta said the Government was "alerted to issues in relation to this minor and made sure that Oranga Tamariki were engaged in the process right from the moment we were notified".
"We were notified that the minor would be returning to New Zealand on March 10. I was notified some weeks ago about the situation. At that point, noting that there were some complex family issues around this matter and that he was a minor, we ensured Oranga Tamariki and relevant agencies would be engaged."
Newshub understands the boy was sent back by himself last week.
The Prime Minister said on Monday afternoon she had just found out about the situation but understood the boy was sent back within a group of '501' deportees.
"Within the group of 501s that we had returned recently, there was what we considered a minor in that group," she said. "I have a specific objection to the fact that we have people being deported from Australia who we consider to be Australians."
Australia's controversial '501 immigration policy' allows Australia to send someone back to their home country if they don't pass a character test - such as going to prison for more than 12 months - regardless of whether they still have ties to that nation.
Mahuta wouldn't discuss the boy's circumstances in detail or if he has lived in New Zealand. He has family in both Australia and Aotearoa, she said.
"Some of the circumstances around his living situation links him to Australia and New Zealand."
The care of the child is critical, Mahuta said.
Green Party foreign affairs spokesperson Golriz Ghahraman reacted to the deportation on Monday night by calling on the international community to treat Australia as a "rogue nation who persistently flouts human rights laws for its racist populist policies".
She said issues like this should be taken to the United Nations.
"I think it is time for us to really act act like we would when a non-Western country does this, displaces people, flouts international human rights rule," Ghahraman told Newshub.
"We need to raise this in international forums and actually call on our friends and allies to get together and to actually see what international sanctions we can put on Australia to pressure them to abide by the standards we expect of any good international citizen."
She said the '501' policy was "cruel" and in line with Australia's other immigration policies, like detaining people on Christmas Island.
"[Australia] sees this as being tough on immigration. The rest of the world has consistently called this breaches of fundamental human rights."
National Party leader Judith Collins thinks the deportation is "pretty bad".
"The 15-year-old, no doubt, has been involved in something they shouldn't be involved in. But actually, you have to be human. As we know in New Zealand, 15-year-olds do get some leeway when it comes to behaviour unless it is extremely serious. I don't know enough about the seriousness of the offending, but I have got to say, it is a pretty tough call."
New Zealand's Children's Commissioner Judge Andrew Becroft says he has "serious questions" about the deportation, but is "proud" Aoteaora is "putting the child first".
"What led Aus to decide his best interests were better served by deporting him here? Was it a principled decision, or just convenient?
"Are the '501 character tests' being applied to children & if so, why? These are questions every New Zealander would want to know the answer to. I certainly do."
A source who was on the flight back to New Zealand noticed the boy on the plane.
He told Newshub he thought the boy was a guard's son, as they wouldn't deport a child. He spoke to him briefly when they were about to land and that he was freaking out.
Oranga Tamariki has confirmed the child is currently in a managed isolation facility and is receiving support while he is in quarantine.
The '501' immigration policy was in the spotlight last week as well after Australia Home Affairs Minister Peter Dutton called a planeload of deportees to New Zealand "trash". Those comments were widely criticised, including by Mahuta who said they only acted to "trash" Dutton's reputation.
The policy has been called "corrosive" to the New Zealand-Australian relationship by Ardern and has been blamed for rising gang activity in Aotearoa.
Ardern says she will continue to raise the issue with Australia.