Statistics NZ is under fire after revealing it only collected individual census forms from two-thirds of Māori.
The data agency on Wednesday said it collected responses from 4.2 million people, despite New Zealand's population expected to reach 5 million by the year's end and it being against the law not to take part. Around 700,000 Kiwis didn't take part.
The rest of the statistics are being filled out with data collected from other Government sources, bringing the total coverage to 98.6 percent.
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Only 83.3 percent of people filled out individual forms, which Statistics NZ called the "traditional NZ method". But only 68.2 percent of Māori did, down from 2013's response rate of 88.5. Even fewer Pacific Islanders did - 65.1 percent, down from 88.3.
The fall in the overall rate - from 92.2 percent to 83.3 - was far smaller.
"The very low Māori response rates mean that a significant share of Māori data in the final census dataset has been pulled from alternative sources," the Māori Data Sovereignty Network (Te Mana Raraunga) said in a statement.
"While that leads to improved coverage rates, it raises important issues about equity, trust and confidence.
"Of the Māori ethnic group counted in the final census dataset, nearly one in four (23 percent) were found in other Government data. That is, the data were pulled from individuals' 2013 census forms or from other administrative data such as birth registrations, health and education enrolments.
"For Europeans, only 8 percent of records were sourced from alternative sources."
The group says this is "unacceptable", as Māori and Pacific Islanders "are among those most impacted by the poor execution" of last year's census.
Census data is used to ensure good decision-making by officials. Statistics NZ acknowledged the drop in responses last year, saying it was "a long-running trend statistical agencies around the world are grappling with".
"Despite this reduction, Stats NZ will produce a high-quality dataset by making use of reliable government data to fill in gaps," deputy Government statistician Denise McGregor said in July last year.
Government statistician Liz MacPherson told Newshub on Saturday she agreed the response rates for Māori and Pasifika were "unacceptably low" and were a "significant concern".
"I deeply regret the low response rates for Māori and Pasifika people. We should have done much better," she said.
"I'm determined that this never happens again. We will do everything we can to ensure that response rates are much higher in the next Census in 2023.
"The 2018 Census clearly didn't work for Māori and Pasifika communities, in particular, and for that I apologise."
Te Mana Raraunga said the shift to an online focus in 2018's census was done "without any substantial input from Māori as Treaty partners".
"With planning for the 2023 census now underway, Māori trust in Stats NZ is likely to be at an all-time low, especially given their failure to produce official Iwi data," it said.
"Te Mana Raraunga looks forward to finding out more about the reasons for the poor census enumeration when the Independent Review report is released in August."
Prime Minister Jacinda Ardern in April blamed the previous National Government for the shambles.