The government's chief statistician Liz MacPherson has finally revealed how many New Zealanders did not complete last year's census.

Photo: RNZ

Ms MacPherson had been threatened with being held in contempt of Parliament, prompting her to write to MPs yesterday to confirm almost one in seven Kiwis didn't complete the census.

Partial responses at the 2014 census were two percent. That number more than doubled to five percent last year.

Statistics Minister James Shaw was backing Ms MacPherson but the opposition said it was losing faith in Stats New Zealand and wanted electoral boundaries to remain unchanged until the next census was complete.

Ms MacPherson had already twice declined to give the information to MPs when National MP Nick Smith had threatened to complain to the privileges committee if she didn't meet the new deadline of today.

In a letter to the select committee yesterday she confirmed about 240,000 people partially completed the census.

That number combined with the approximately 10 percent of people who did not participate at all would mean almost 700,000 people did not complete it.

Dr Smith said it was one huge data hole when it came to fairly allocating tens of billions of dollars for public services.

"They said it was all hunky dory and going well. Minister James Shaw claimed it was going to be the best census ever - that was not true and if they'd listened to some of that criticism we wouldn't be in the mess we now are," Mr Smith said.

But Mr Shaw is standing by Ms MacPherson and said she'd delivered what was asked of her.

"The government statistician is statutorily independent and has considerable independence over what she reports on, how the methodology is formed, the timing she releases statistics on. She actually has delivered on the questions she was asked last week."

Photo: VNP / Phil Smith

He didn't find the lack of data concerning, but said he found Mr Smith's campaign to undermine public confidence in the national statistics system concerning.

Dr Smith, however, is fed up with the lack of information coming his way from Stats New Zealand.

"Getting information out of Statistics New Zealand about the census has been like drawing teeth. It has taken Parliament quoting standing orders and contempt proceedings to finally get the chief statistician to provide information on the number of complete responses. They have, as expected, shown a bleak picture,'' he said.

National Party leader Simon Bridges said he could no longer rely on the numbers Stats New Zealand are providing.

"I'm losing faith in Stats New Zealand. I think it's a situation where we've got a census now we have a huge amount of doubt in."

But Prime Minister Jacinda Ardern said it was a bit rich for the opposition to be up in arms about the census failures given the decisions made by the last National government to move to an online format.

"I'd have to say I don't think it's a fair criticism for a member of the past government who made the decision to move to an online census, who made the decision that they should reduce their funding to then criticise the consequences of those decisions,'' she said.

Photo: RNZ / Rebekah Parsons-King

At the Iwi Chairs Forum at Waitangi in February the holes in the data from the census were raised as a significant concern for Māori, in particular, how it would affect the number of Māori seats and a raft of other public services like health and education.

Mr Shaw has subsequently met with iwi leaders to try to alleviate their concerns but has stopped short of giving an assurance that Māori won't be left out of pocket.

"My point is that it's the job of the government statistician and Stats New Zealand under the powers they have to ensure the country has high-quality data. I'm absolutely convinced they're doing everything in their power and working as fast as they can to produce the same or better quality data as a result of this year's census as they would for any other census," Mr Shaw said.

Mr Smith said Māori, the elderly and people living in remote communities will be most affected by the missing data and Māori were right to be concerned about the impact on electoral seats and public funding.