Fewer New Zealanders than expected completed this year's census after the process was moved online, Stats NZ has revealed.
The online system was shut down last week and interim calculations for this year's census showed Statistics New Zealand received full or partial information for at least 90 percent of people, compared with 94.5 percent during the last census in 2013.
But government statistician Liz MacPherson said more than 82 percent of responses were online, which surpassed its 70 percent target.
She said while parts of the census operation worked well, there were some aspects which proved more challenging than anticipated.
"It is normal for some people to not fill in census forms, or submit their forms with unanswered questions, but this year we haven't received as many responses from individuals as we expected."
She said the lower response meant it would take longer to draw on other information sources and new methods to achieve a high-quality dataset.
"We're well placed to do this as we have been developing new approaches for future censuses for some time, in particular by using the other government information that we hold to complement the data we collect through the census."
Ms MacPherson said Stats NZ would look at the reasons behind the lower response in its review.
"We already know we didn't get everything right. We built new systems and processes to run this census, and while the majority of New Zealanders were able to take part without a hitch, we know that some people did not have a good experience this year.
"I have had mixed feedback from people. For some it was the easiest census ever; for others it has been a frustrating experience. For that I am sorry."
Grey Power Otago president Jo Millar said the figures didn't surprise her.
"We had a lot of people who were completely frustrated over the census situation, not only the ones who didn't have access to computers, but the ones who did have access found it quite awkward to work with," she said.
She said many people told her taking away the human element and lengthening the time to complete the census, made it feel like it wasn't important enough to do.
Ms MacPherson said they put in a number of approaches, such as working with councils and libraries and even set up a caravan for people to fill in their census in remote areas, but added they may need to do more next time round.
She hoped New Zealand would be an even more digitally inclusive society by the next census in five years time.