Six Northlanders are being prosecuted for failing to fill out their Census forms, but more than 700,000 Kiwis did not complete last year's national population count.

Statistics New Zealand will be lodging 60 cases throughout the country for people who deliberately did not fill out the Census and has already filed six in Northland in respect of the 2018 Census.

Under the Statistics Act 1975, everyone must fill in a Census form and people can be fined up to $500 per charge if they do not participate or they provide false or incomplete information.

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.

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."

Smith 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.

Of the six cases in Northland being prosecuted, one has been heard in the Kaitaia District Court while the remaining five will be dealt with in the Whangārei and Kaikohe courts.

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"Court proceedings have only recently begun, so there have been no convictions as yet or cases withdrawn. While it is a legal requirement to take part in the Census, it's not practical to prosecute everyone who hasn't taken part," said the Stats NZ general manager Census, Kathy Connolly.

She said cases have been prioritised based on a number of criteria, including whether there was an active refusal to take part in the Census or a strong negative or abusive attitude displayed towards Census staff.