On Wednesday, Stats NZ revealed census information had come from only 90 per cent of the population - down from 94.5 per cent at the previous census in 2013.

If you're among the one in ten who didn't fill in their census form, Statistics New Zealand still has the option of prosecuting you.

Stats NZ reported on Wednesday census information had come from only 90 per cent of the population - down from 94.5 per cent at the previous census in 2013.

The release date for the first information from the census that was held in March has been pushed back six months from October 2018 to March 2019.

National state services spokesman Nick Smith has gone as far as to raise the possibility of holding the census again because of the low participation rate. National wants the State Services Commission to investigate.

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When asked what had gone wrong with the census, Stats NZ did not answer the question.

Instead, it said: "We're pleased with the online response and performance of the online system for 2018 census. Eighty two per cent of responses received were online which surpasses our target of 70 per cent.

"We will be undertaking a full review of the 2018 census including the planning and operational phases which will identify what areas of the census could be improved for the future."

STATS NZ Flow of 2018 census data through the processing system.

Stats NZ's website shows people who refused to complete the census forms, or filled in the forms in a non-compliant manner would be sent two reminder letters.

A total of 1,124,659 reminder letters were sent to non-responding households, Stats NZ said on Thursday.

If people didn't respond to the letters they would get a non-response follow up visit, as would people who hadn't filled out their form after it was delivered to their address.

Field officers started follow-up visits on March 16.

"Over the course of the visit phase, field officers recorded 980,175 visits to 595,907 households and delivered 512,835 paper form packs," Stats NZ said.

"They made between 1-7 visits per household; with all non-responding households receiving at least 1 visit. On average, a household had 2.27 visits during the visit phase, which was in line with expectations."

A visit did not necessarily result in a census response.

"If a person remedies their non-compliance prior to the filing of charges the Prosecutions team may decide that no further steps will be taken," Stats NZ said. The context of each specific case, including the timeliness of compliance, would be considered.

At the end of the "compliance encouragement stage" all those who continued to refuse to fill in the form would be assessed by the prosecutions team.

PROSECUTION PROCESS

"The prosecution process involves a number of steps. Our next step will be to send Notice of Liability to Prosecution letters to 238 people on Monday, 16 July 2018. We will not know how many prosecutions are planned until we have received any responses to those letters, Stats NZ said.

The prosecutions team would consider whether there was strong admissible evidence of committing an offence, and identify which offence was the most appropriate to form the basis for charges.

Action wouldn't be taken against people if the prosecutions team had reason to question their mental capacity, or their wellbeing.

A statistical analyst in the census team would independently peer review the cases assessed as appropriate for prosecution.

"It is expected that there will be a higher number of those who refuse than is practical to prosecute. The decision on the total number of prosecutions will be made by the chief executive of Stats NZ (or a nominated delegate, eg a deputy chief executive)."

Priority would be given to prosecutions where a strong negative attitude was exhibited, particularly toward Stats NZ staff or contractors. The number of people affected by the non-compliance would be considered, along with whether the person involved had encouraged others to commit an offence against the Statistics Act.

Prosecution would only be initiated if both limbs were satisfied - the evidence provides a reasonable prospect of conviction, and prosecution was required in the public interest.

"The primary objective of prosecuting for breaches of the Act is to achieve compliance through general deterrence, thereby enabling greater compliance in any subsequent census," Stats NZ said.

CONFIDENCE LOST

National's Smith told Radio New Zealand concerns raised about the census during the past four months had been dismissed.

"We've lost confidence in Statistics New Zealand," he said.

"We would go so far as to say that if this data cannot be relied upon, then we need to leave the door open to re-doing the census, given the tens of billions of dollars, and important issues like setting electoral boundaries, that flow from this," Smith said.

"The State Services Commission needs to get to the bottom of what has gone wrong here. because of the flow on implications, and if there is some blame that lies with National so be it, and let that be independently explored."

INDEPENDENT REVIEW

The 2018 census process had been discussed with international experts including the Unite States Census Bureau and the United Kingdom's Office for National Statistics, Stats NZ said.

It was also setting up an independent panel of data and statistics experts to assess the census results as they come out and to give confidence in the results.

"In addition, we will have a full independent external review and will work closely with the State Services Commission on this, including the development of the terms of reference,

"This review will be undertaken once the census dataset has been completed and the findings of this review will be reported to the Government Statistician and to the State Services Commissioner."

STATS STILL CONFIDENT

Stats NZ is still confident the 2018 census will deliver reliable data.

Despite the lower response rate, it would "produce a high-quality dataset by making use of reliable government data to fill in gaps," deputy government statistician Denise McGregor said.

That method has been used by Stats NZ to fill in census gaps since 2001.

Other national statistics organisations around the world did the same, Stats NZ said.