Statistics Minister James Shaw has warned National to "back off" their attacks over the 2018 Census.
For the first time last year, the Census moved from paper copies and door-to-door collection to online.
As a result, Stats NZ says around 700,000 people did not complete the Census. One year on, National says there's a huge data hole.
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"That creates enormous problems for tens of billions of dollars in funding for health, for schools, for police, for sport, all of which depends on reliable population numbers," National's State Services spokesperson Dr Nick Smith says.
It also affects electoral boundaries, the number of Maori seats and general seats for next year's election.
But the Government says Dr Smith is scaremongering, and the data will be more complete than they say.
"The reason why Stats are taking their time over Census data is to make sure we get as good, if not better information from other Census," Shaw says.
"Dr Smith is way out of line and he needs to back off."
On Tuesday, Prime Minister Jacinda Ardern hit back at Dr Smith, pointing out that it was the previous Government that made the decision to move to an online Census - which appears to have been part of the problem with the big drop in responses.
The chief statistician is also missing. Liz McPherson refused to answer questions on Wednesday about why so few Kiwis took part.
She has been threatened with contempt of Parliament for denying basic Census information.
She declined to go on camera on Wednesday, instead saying she'd hold a media conference at the end of the month where she'll say more.
"The Government statistician has a statutory obligation to protect data, it's one of the reasons New Zealanders trust Stats NZ," Shaw says.