New Zealand First leader Winston Peters, known for pushing for tighter immigration controls, says it was a "responsible" decision to grant residence to drug-smuggler Karel Sroubek.
And he is backing Immigration Minister Iain Lees-Galloway, who granted residence despite Sroubek being found guilty of using a false passport and being in jail for smuggling MDMA.
The National Party has been calling for Lees-Galloway to explain or resign, and has challenged the credibility of the minister's information, and whether Sroubek's life would be in danger if he were deported.
Peters said he had been briefed on the decision and backed Lees-Galloway.
"I think the minister was forced inevitably to make this decision. It wouldn't have been easy, but he's made it and we stand behind him.
"The minister has made a responsible decision and has put serious caveats on that decision going into the future."
Prime Minister Jacinda Ardern has also backed Lees-Galloway, who said that public safety was a chief concern when he was making his decision.
"I can assure people that I weighed public safety very heavily in the decision I made," Lees-Galloway said.
"I can understand why people want an explanation, but for legal reasons and to maintain the integrity of that process, I cannot and will not divulge the information I used to make that decision."
Ardern said that previous media coverage of Sroubek's entry to New Zealand provided hints behind the minister's decision, but would not elaborate.
Sroubek came to New Zealand from the Czech Republic in 2003 after fleeing corrupt police, but was unmasked in 2009 when New Zealand police were alerted to his identity and an international arrest warrant.
He was found guilty of using a false passport in 2011, but was discharged without conviction after Judge Roy Wade said Sroubek would be in danger from corrupt Czech authorities if he were deported.
But Gregory Shanahan, the Czech Republic's Honorary Consul in New Zealand, seemed to downplay the possibility of corrupt Czech officials.
"It has a well-established and rigorous legal system and police force, and other forces which comply with what everyone understands to be international codes of practice in these areas," Shanahan told the Herald.
"It's a member of the European Union. I have every confidence in the integrity of the Czech legal system and the people who are charged with its enforcement."
National Party leader Simon Bridges said that he would have deported Sroubek.
He questioned the credibility of the information the decision was based on.
"They've been sold down the river with a tale that immigration officials and courts get every day of the week. They're being naive."
National's justice spokesman Mark Mitchell said he was prepared to travel to the Czech Republic to find answers.
"Some of the information I have would show that the threat to him is probably his own mates, because he's been involved in organised crime in the Czech Republic. Actually, that doesn't qualify you for New Zealand residency."
Sroubek already had residence but it had been in the name of Jan Antolik, the name he used on his false travel document.
Lees-Galloway's move gave him residence in his real name, conditional on him not being convicted of any offence, not using any fraudulent identity, and not providing false information to a Government agency for the next five years.
In 2016, Sroubek was jailed for five years and nine months after being convicted of importing 5kg of MDMA with a street value of $375,000.
Last month the parole board noted he was a low risk of reoffending, but denied parole due to Sroubek's criminal associations in the Czech Republic, with Hells Angels, and because his interview responses were "self-exculpatory, evasive, long-winded and ultimately in our view in many respects manifestly untruthful".