The head of Statistics New Zealand has revealed that one in seven Kiwis did not complete last year's census – the worst turnout in decades.

Stats NZ's chief statistician Liz MacPherson yesterday wrote to MPs, after being threatened with a complaint to Speaker Trevor Mallard if she did not deliver the census information to a committee of MPs by today.

She has twice refused to provide Parliament's governance and administration committee with the number of partially and fully completed responses to the 2018 Census.

It was revealed that 700,000 New Zealanders had either not participated, or not completed the census.

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Minister of Statistics James Shaw told RNZ yesterday MacPherson had delivered on what she had been asked to.

"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," he said.

In the letter to MPs, MacPherson said the issue was not as straightforward as it may appear and said there had been "considerable misunderstanding" about last year's census.

"This is because the way in which census forms have been designed for many census cycles means that people are only required to answer the questions that are relevant to them."

Some questions, for example name, age, address and ethnicity, are relevant to everyone.

But that is not the case for every question.

For example children only answer about half of the census questions and overseas visitors only answer the first nine questions, MacPherson said.

In the letter, she said the more details around the timing of the census information would be released publicly on April 29.

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According to Stats NZ's interim calculations, roughly 10 per cent of New Zealanders didn't properly fill out their census forms – constituting what would be the lowest participation of the past five surveys.

Some 460,000 people did not complete the census and roughly 240,000 people only partially completed it.

It has taken longer than in previous years for the information to be reported back to the public.

National's state services spokesperson Nick Smith said the fact that so many people did not fill in the census was problematic when it comes to budget allocations.

"This leaves a huge data hole that will create problems for years in allocating tens of billions of dollars in funding for central state services like health and education, as well as affecting electorate numbers and boundaries for Election 2020."

He said the problems with Census 2018 were so bad that consideration should be given to deferring the electoral boundary changes for 2020 and bringing forward the next Census to 2021.

National leader Simon Bridges was also clearly frustrated when talking to media yesterday.

"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 pointed the finger back at National and said it was the decision of the last government to move the census online.

"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."