Prime Minister Jacinda Ardern is unhappy with the process that saw Karel Sroubek granted residency and wants it improved, but is standing by her Immigration Minister.

She told Newstalk ZB this morning that she still had confidence in Iain Lees-Galloway and was not expecting a resignation from him, nor would she accept one.

But she does not have confidence in the decision, which is being reviewed by Immigration NZ in light of contradictory claims that, if true, would undermine why convicted drug-smuggler Sroubek was granted residency in the first place.

National has renewed its calls for the axe to fall on Lees-Galloway after he admitted he didn't read the whole case file and made his decision in about an hour.

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"That's not careful consideration of what was a dangerous decision, and it is not acceptable due diligence from a senior Cabinet minister," National leader Simon Bridges said.

"The Prime Minister cannot expect the public to have confidence in any of his decisions given his careless approach to Sroubek's residency."

Asked if she would accept Lees-Galloway's resignation, Ardern told Newstalk ZB: "No, because at the moment what I'm focused on getting this issue right."

She said that the process had existed across successive ministers and governments of different political colours, and case files were often hefty documents with a significant amount of supporting information.

"I'm not saying that I'm happy with this process or with this outcome. Obviously I'm not happy with this situation.

"I'm not happy with the decision, given there is contradictory information in the public domain that wasn't contained in the report generally, or the supporting evidence.

"That to me is key. That's why Immigration [NZ] is going back and doing additional work on this case."

She indicated that the process wasn't good enough.

"We do need to look at the process generally around deportation orders.

"This is not unique to our Immigration Minister. Successive immigration ministers have gone through a process with Immigration [NZ] that I think can be improved."

Lees-Galloway maintains that he was thorough in making his decision, and has since gone back, read the entire case file, and stands by the decision based on the information he had at the time.

"I read various aspects of the full file. I didn't rely solely on the summary. This is the usual process for these decisions," he said yesterday.

"I took much, much longer on this decision than I have on other decisions, and I'm following exactly the process that I inherited from the previous minister."

Immigration NZ general manager Stephen Dunstan said the process for absolute discretion by a minister has been in place since 2003, and it was not unusual for decisions to be made on the same day that the files arrive.

He said a case file could be up to 100 pages long, depending on how complex it was.

"The time-frame is really up to the minister. Some decisions are made on the same day. Sometimes ministers might ask questions or request further information," Dunstan said.

National has continued to press Lees-Galloway in the House, and he has repeatedly declined to answer questions about the Immigration NZ review, saying he does not want to prejudice the review or any legal action that might follow.

On Thursday, National's deputy leader Paula Bennett used parliamentary privilege to reveal an allegation that Sroubek threatened his estranged wife in a phone call from prison in May this year.

On Wednesday, she used Question Time to expose an alleged burglary of a $2.3m house just days after Sroubek placed a caveat on it, and on Tuesday she referred to court documents to reveal that a family was placed in a witness protection programme because of Sroubek's alleged actions.

Sroubek released a statement through his lawyer, saying he had nothing to do with the alleged burglary and was acquitted in the trial that involved the witness protection programme.

"Comments made about that case in the media are not balanced, and in particular do not reflect that the key prosecution witness' evidence was discredited," Sroubek said.

"Much of what has been said about me and my circumstances does not present the true picture.

"Until Immigration NZ reports backs to the Minister and I have had the opportunity to respond to him on any issue he may wish to raise, I will be making no further comment or statement."