National say the low census turnout raises "serious questions for the Government to answer" and that the drop in numbers could lead to a compromise in the statistics.
"Census 2018 has turned into a shambles with the lowest participation rate in over 50 years," National's spokesperson for State Services Nick Smith said. "The Government must figure out what went so wrong and how the serious flow on problems for the public sector can be rectified," Dr Smith said.
Statistics New Zealand interim results showed information for 90 per cent of the population was collected, a drop from the 94.5 per cent in the 2013 census.
Acting Prime Minister Winston Peters was asked today about the census levels, saying he had "no idea" why the turn out was so low.
"It's very disappointing given that it is very important we have accurate data to be able to forecast what the future population might be and the needs might be."
He said filling out the census was a "civic and social responsibility issue".
When asked if he thought the digital-first approach would have impacted numbers, Mr Peters said "you see on television people walking 10, 15 miles in very difficult environments overseas just for the right to vote, we've just got to go down the road, grab the form and do the job".
"How can it be that difficult?"
Dr Smith said the 400,000 people who did not complete the census would "will compromise the quality of the statistics".
"Any assumptions about the make-up of the 400,000 will significantly distort the statistics. Changes in population figures as small as one per cent can impact on whether there is, for instance, an extra or the removal of one of the Maori electorates."
"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' success."
In New Zealand law, Statistics Act 1975 requires New Zealanders to fill out the census, with fines between $50 and $500 possible.