Last year's census undercounted by nearly 104,000 people the number of New Zealand residents in the country at the time, Statistics New Zealand says.
SNZ 2013 census general manager Gareth Meech said some groups were becoming much more difficult to reach.
"It's fair to say it's harder to find people," he said.
"People are in and out of the house at different times, sometimes ... families are split over two or more households."
Some people also did not fill in their census form due to apathy, others were concerned about providing information, and some people did not believe in the census and did not want to contribute to it.
If the occupant of a house actively avoided a census collector who visited five or six times, it was hard to know why that person did not fill in their form, Meech said.
While the national net undercount in the survey was 2.6 per cent, the figure for Maori was 6.1 per cent, for Pacific peoples it was 4.8 per cent, and for those aged 15-29 it was 4.8 per cent.
Those groups traditionally had a higher undercount, Meech said.
On census night, 4.24 million people were counted as usually living in New Zealand and being in the country at the time. That figure is called the census usually resident population.
It is lower than the estimated resident population which, as well as adjusting for those missing from the census, also includes residents temporarily overseas at census time.
SNZ calculated the size of the undercount by carrying out a survey after the census in which about 15,000 households were scientifically selected and surveyed.
The undercount was made up of 135,500 people who were missed out, minus 31,700 people counted more than once.
SNZ said the survey showed the 2013 census, carried out in March, counted 97.6 per cent of residents in the country on census night, referred to as the coverage rate.
Another measure - the non-response rate - was 7.1 per cent. It is made up of the net undercount, plus those who were counted in the census but for whom no form was received.
SNZ had failed to meet its ambitious targets of 98 per cent for coverage and 95 per cent for the response rate.
Despite that, it was confident the census had produced high quality timely information. The modernised 2018 census would make it easier for people to take part, with the aim of improving both coverage and response.
The first population estimates with the 2013 census as a base will be released in August.
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