The Immigration Minister shut down a category of immigration last year on the basis of incomplete information, it has emerged.
Michael Woodhouse temporarily stopped letting immigrants' parents make residency applications in October.
Under the parent category, migrants have to agree to financially support parents for five years if they move to New Zealand. Mr Woodhouse said too many were failing to do so and this was costing the taxpayer tens of millions of dollars.
But a report, released to RNZ under the Official Information Act, shows less than 2 percent of parents were taking up welfare payments.
The report to ministers detailed the findings of a four-year study that showed only a small number of migrants failed to meet their obligations.
It looked at more than 14,000 parents and found only around 250 had accessed support between 2010 and 2014.
However, Mr Woodhouse was not aware of that report when he made his decision.
He said he did not know about this report at the time, and relied on other information that showed a higher rate of migrants not meeting their obligations.
"Notwithstanding the sponsorship obligations their children had made, more than 11 percent of them were on a benefit at two years after arriving and nearly half of them were on benefits five years after arriving," Mr Woodhouse said.
"It also said health care costs for those people were three times higher than equivalent aged New Zealanders."
Mr Woodhouse could not explain why he had never been given the report RNZ obtained.
"Well that's a question you should put to the ministry," he said.
"I would want to get more detail about that because it did seem inconsistent with the previous advice that I had."
But Mr Woodhouse said the new information would not have changed his decision.
He said the parent category probably would not open for new applications until sometime next year.
And when it does, migrants will have to sponsor their parents for 10 years.
Immigration crackdown dog whistle politics - Green Party
Green Party immigration spokesperson Denise Roche said she was not surprised and the decision to crack down on immigration was always dog-whistle politics.
"My understanding is that they shut down the parent category in order to be seen to be doing something to reduce the number of migrants coming into New Zealand.
"But actually all it's done is it's made it harder for those people that we do want to live here to settle here."
New Zealand First leader Winston Peters said he did not have much faith in the accuracy of the report showing only a small number of migrants failed to meet their parental sponsorship obligations.
"With the greatest respect that's bulldust. We're becoming a retirement home for those sorts of applicants.
"And that's why the Chinese and the Indian spokespeople were so worried about what the National Party has just announced, and they wouldn't be making those comments, if it wasn't being accessed."