Statistics New Zealand is prosecuting 60 people for refusing to fill out last year's Census.

Yesterday it was revealed that one in seven Kiwis had failed to complete the national population assessment - but only 60 will face legal action.

Everyone must complete a Census form and those who don't or who provide false or incomplete information can be fined up to $500, under the Statistics Act 1975.

There's an ongoing liability created by the act whereby a failure to complete the form after conviction may render people liable to a fine of up to $20 a day while the forms remain incomplete.

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However, Stats NZ's chief statistician, Liz MacPherson, on Tuesday wrote to MPs to confirm that more than 700,000 New Zealanders had either not participated, or not completed the Census.

Read more: Head of Stats NZ has told MPs that one in seven Kiwis did not properly complete the Census

This comes after MacPherson had been threatened with contempt of Parliament if she did not deliver the Census information to MPs by yesterday.

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.

MacPherson said 460,000 people did not complete the Census and roughly 240,000 people only partially completed it.

This Census was the first to be completed online and it has taken longer than in previous years for the information to be reported back to the public.

The National Party's state services spokesman, Dr Nick Smith, said the fact 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."

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