Prime Minister Jacinda Ardern is backing her Immigration Minister over the granting of residency to Czech drug-smuggler and gang associate Karel Sroubek.

And while she won't discuss details of the case, such as whether Sroubek's life might be in danger, she says the decision not to deport him is based on more information than simply Sroubek's word.

The National Party has called on Immigration Minister Iain Lees-Galloway to resign if he cannot justify his decision to grant a residence visa to Sroubek, even though Sroubek is serving a jail term for drug-smuggling and was convicted of using a false passport in 2011.

Speaking to Mike Hosking on Newstalk ZB this morning, Ardern said she knew the rationale behind Lees-Galloway's decision and supported the minister.

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"We are a country that supports things that often lead to difficult decisions. Things like human rights ... It's not an easy decision.

"If it was, we would have simply deported him. There are other things at play that weighed on the minister's mind. He's had to make a tough call."

She rejected comments from National's immigration spokesman Michael Woodhouse, a former immigration minister, that he would have sent Sroubek packing.

"He doesn't have the full information in front of him, in the same way we don't have the full information of the 108 deportations he cancelled. You have to rely on ministers who do have access to the full information."

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 - starting from when Sroubek is released from prison.

National's justice spokesman Mark Mitchell told Newstalk ZB that Lees-Galloway's refusal to front with the details of his decision were on par with his arrogance.

"We've noticed in the House is that he's certainly showing a high level of arrogance and contempt ... His actions now are completely consistent with that level of arrogance.

"They're trying to hide behind this cloak of invincibility by saying somehow he's at risk if he goes back to the Czech Republic. I don't believe they've done any proper investigation around that."

Mitchell said he was looking into going to the Czech Republic to get answers if they are not forthcoming from the Government.

Sroubek came to New Zealand from the Czech Republic in September 2003 to start a new life as Jan Antolik, after fleeing corrupt police who wanted him to lie and clear the main suspect in a murder investigation.

Instead, he left a videotaped witness statement which was later crucial in convicting the killer.

He fled the Czech Republic with a doctored passport but was unmasked in October 2009 when Czech police gave New Zealand police details of his identity and an arrest warrant on minor charges in connection with the 2003 murder.

Sroubek was discharged without conviction after being found guilty of using a false passport in 2011. Judge Roy Wade said Sroubek would be in danger from corrupt Czech authorities and the man he helped convict of murder if he were deported.

Mitchell did not believe that was still the case, saying the Czech Republic - an EU and OECD country that is also a member of NATO - had a first-world justice and police system.

Ardern said the information in front of Lees-Galloway was more than simply Sroubek's own claims.

"There is other information provided. Of course, you can't simply make decisions based on someone's word.

"No one is sitting here applauding the fact we're in this situation. I'm defending the fact that we have a set of rules in place ... that allows one person to have everything in front of them. That weighty job sits with Iain Lees-Galloway."

Sroubek was jailed in 2016 for five years and nine months after being convicted of importing 5kg of MDMA with a street value of $375,000.

He appealed against that conviction last year, saying the MDMA was planted to frame him. The Court of Appeal rejected his appeal.

Sroubek was also previously acquitted of committing an aggravated robbery with two members of the Hells Angels.

Ardern said Sroubek was now on "absolute notice".

"What happens to him in the future is not on the minister's, of the Government's, or New Zealand's conscience. It's on his."