Just 90 percent of New Zealanders filled out the Census in 2018, leaving a data hole some several hundred thousand people deep.
It's the lowest rate of participation in five surveys.
Minister and Statistics NZ James Shaw said they are both concerned about the lack of response.
It was the first time Statistics NZ had adopted a 'digital first' approach to the Census, which is a thorough collection of data that takes place every five years. It's used to help Government, business and iwi make decisions about funding and projects.
"A review of the Census will look to understand why that has happened and take lessons that can be applied in the future," a spokesperson for Mr Shaw told Newshub.
When asked whether Mr Shaw should have focused more on publicising the Census, the spokesperson said "there was a well-planned roll-out of publicity by Stats NZ across multiple media formats, including social media".
The low participation rate means the statistics won't be available until March 2019. It was due for release in October.
Statistics NZ will have to use data from other agencies, such as Inland Revenue, the Department of Labour, the Ministry of Education, Health, Immigration and Department of Internal Affairs, to help plug the data gap.
"Similar methods have been used in the last three census cycles, including the 2013 Census, and are international statistical best practice," Stats NZ Said.
"Stats NZ is confident its final combination of data will provide robust information that is as accurate as possible."
Opposition spokesperson for state service Nick Smith says making assumptions about the missing 400,000 New Zealanders will distort the statistics.
"The Minister and Chief Statistician must accept responsibility for this debacle. They rejected serious concerns about the excessive reliance on online census returns, repeatedly reassuring the public of the census's success," Dr Smith said.