Simon Bridges says he's losing faith in the accuracy of the official statistics Parliament relies upon to make big decisions.
Statistics NZ chief executive Liz MacPherson is refusing to release information collected during last year's census, saying they need more time.
The census - New Zealand's first to be carried out largely online - has been roundly criticised as a shambles, with an estimated 10 percent of New Zealanders failing to take part despite it being compulsory. Many others are believed to have only partially filled out their forms.
"If you can't get the numbers right about basic stuff, you can't do anything," Bridges told The AM Show on Monday. "You don't know where to put your health money, your education, you name it."
The first figures were supposed to be out in October, then March, and now MacPherson is refusing to release anything until the end of April.
"It is my hope that the committee will appreciate that I have made this determination after careful thought and application of statistical best practice," MacPherson told NZME last week, saying a premature release of figures without context would be "open to misinterpretation".
National MP Nick Smith has threatened to report her to the privileges committee for contempt of Parliament if she doesn't hand over information by April 10.
"I'm starting to lose faith in our statistics and our statistician for New Zealand," said Bridges, National Party leader.
"Because look, you just say you've had a situation where you've got a census botched. You've also got immigration numbers, GDP numbers, the big basic stuff, where they keep chopping and changing. I'm not sure what the real numbers are... There's all these partially completed census forms... not completed ones, all this stuff. How can we have faith in the numbers?"
Bridges said the delay will have an impact on how well the Government can allocate resources in the upcoming Budget, which Labour says will focus less on traditional metrics like GDP and more on outcomes such as "improving child wellbeing".
"If they can't get the basics right, good luck with the wellbeing stuff," said Bridges, adding that he's "always been a sceptic of online".
"I don't believe in it for voting. I think you end up with all sorts of problems."