The Government has been accused of failing to hold charter schools to account by blocking a plan to compare their results with those of state schools.
Opposition MPs said the Government was throwing large sums of cash at charter schools and should be demanding that information.
The Education Ministry offered to do the comparison between charter and state school students in March, because it said such comparisons had been made in other countries.
Official documents show Education Minister, Hekia Parata, vetoed the plan because asking the schools to jump another hurdle shortly after they had been set up was not in students' best interests.
Charter schools, which are privately run but publicly funded, first opened in New Zealand last year.
Labour's education spokesperson Chris Hipkins said the plan would not have required charter schools to jump through any hoops.
"They've been funded at a significantly higher rate. It simply defies belief that Hekia Parata would block the Ministry of Education doing a pretty basic comparison of their results compared to equivalent state schools."
"It means that they're not being held to account. They're not being accountable at all in fact," he said.
Mr Hipkins said the minister may have stopped the work being done because she knew the results would not be impressive.
New Zealand First education spokesperson Tracey Martin said she was not surprised the minister did not want to know the results.
She said there were early indications charter schools were a failed experiment and were not doing any better than state schools.
A spokesperson for the minister said there was a wealth of information publicly available about the performance of charter schools, so people could make their own comparisons.
The Parliamentary Under-Secretary to Education Minister David Seymour said the minister was right in delaying the work.
He said a comparison would be done once the schools had been operating longer.
"There will be an apples for apples comparison of the performance of partnership schools. The only questions are exactly how that gets done and when."
"It's going to take some time to have enough data because the kids have only been there for a bit over a year so far."
Mr Seymour said charter schools were a pilot project and they would be rigourously tested.
An education expert said the success of charter schools could not be judged, because their evaluation was not vigorous enough.
Professor John O'Neill Director of the Institute of Education at Massey University said without results or robust evaluation no one could know if the schools were working.
He said researchers needed to spend weeks or months in schools, instead of one day.