Immigration Minister Iain Lees-Galloway says he regrets not asking more questions in regards to the controversial Karel Sroubek case before making his initial decision.
It was revealed Mr Lees-Galloway was informed of Sroubek's crimes in the Czech Republic but allowed him to stay in New Zealand anyway.
Mr Lees-Galloway over-rode that original decision yesterday and ruled Sroubek could be deported once he was released from prison.
Sroubek was "liable for deportation" because his Czech convictions meant he should not have been allowed in New Zealand in the first place, he said.
Mr Lees-Galloway told Morning Report he would have made a different decision the first time around if he had obtained all the information which he has today.
"If I had the information available that I had when I made the second decision, I would've made a different decision."
Watch Immigration Minister Iain Lees-Galloway on Morning Report:
However, he said he did ask Immigration NZ further in regards to the point in Sroubek's case file which stated he had travelled in and out of the country multiple times since his first recorded arrival in 2003 - despite Sroubek saying his return to the Czech Republic posed a risk.
"That absolutely undermines his case that his personal safety was at risk in the Czech Republic," Mr Lees-Galloway said.
"What I was told was that that it was not information that Immigration New Zealand recorded or had available to them and that it would be information that they would unlikely to be able to access."
But he admitted that he should have investigated further.
"I do take responsibility for the fact that I didn't push harder on the question of whether he'd been back to the Czech Republic. I should've asked more questions, I absolutely take responsibility for that.
"I certainly regret not asking more information about his travel back to the Czech Republic."
In the minister's decision to allow Sroubek to stay, he said he also considered the judge's determination that a conviction - would have come with a deportation liability - posed a significant threat to Sroubek's personal safety.
"I have to weigh all the information that I had, there's a variety of factors at play here, I can't just pick out one factor and say I'm making my decision on that," Mr Lees-Galloway said.
"I had to weigh his offending, and absolutely his offending was deeply concerning.
"I had information in that file that said he had not committed any violent offences in New Zealand, that he had no gang connections, and that he presented a low risk of reoffending."
At the same time, the minister said he also considered that safety risk argument by Sroubek might be embellished
"I wasn't totally convinced that returning to the Czech Republic posed a risk to his safety but I determined that there was a significant enough risk that I had to consider it."
A review into the Immigration NZ process and compilation of material provided to the minister was now under way to ensure correct decision-making, he said.
"One of the things that Immigration New Zealand has decided to do is review their processes for putting this information together to make sure that they are providing me with all the information that I need."
Meanwhile, National Party's immigration spokesperson Michael Woodhouse is calling for the minister to resign to protect New Zealanders from the possibility of what he calls another dangerous and inept decision.
Mr Woodhouse said he was dumbfounded at the minister's decision-making and unable to fathom giving his successor any advice.
"That's really a hard question to answer... because I'd never find myself in this situation. And he can talk about the new information and questions about the process all he likes but that's a straw man argument that covers the fundamental point that he made a shocker."
Mr Woodhouse said it came down to incompetence and not faulty processes.
"If he can't even get the compelling cases right... it really does show a gross ineptitude that I just can't see him coming back from."
Sroubek came to New Zealand under a false identity which is why his Czech convictions went unnoticed by Immigration NZ when it approved Sroubek's temporary and permanent residency visas.
Sroubek's lawyer Paul Wicks QC said his client was disappointed with the decision and planning on appealing the decision through the Immigration and Protection Tribunal.
Sroubek was also weighing up a judicial review, he said.