The Prime Minister says she does not want a review into the decision not to deport convicted Czech drug smuggler Karel Sroubek to turn into a witch-hunt.
Jacinda Ardern told Morning Report she wants to get the case right, before asking questions about what went wrong.
Sroubek is at the centre of a controversy after Immigration Minister Iain Lees-Galloway stopped him from being deported.
Officials are now reviewing fresh information on the case that appears to contradict information given to Mr Lees-Galloway, and said the review would take three weeks.
Ms Ardern said evidence that is counter to the information used to make that decision has now emerged.
"Here I am not placing blame, because there seems to be a range of issues here at play with this case and until we have all the evidence in front of us I do not want this to turn into a witch-hunt, that's not what it should be."
Ms Ardern said ministers could only make decisions based on the information that was in front of them.
"I don't that think anyone would assumes that a minister will for instance receive a significant file and on the back of that go and ask questions - yes they ask questions, yes they scrutinise things, but it's always based on what's there in front of them."
She said in this case there appear to be contradictions that Mr Lees-Galloway could not have known existed.
Ms Ardern said before looking at future processes for dealing with deportation, it was first important to get this case right, and to investigate whether there was information that was contradictory, not made available or missing.
"There is an issue around whether or not the process for these decisions, whether or not that can be beefed up but I want to focus on getting this case right before we go down that track."