If that number is exceeded, "respondents will receive a notice advising that the site is busy. The site will be limited to a maximum number of users to ensure the site provides a good user experience," she said.
"If our 'site busy threshold' is reached, those users already on the site will be able to complete their forms successfully. It is only new users that will receive a busy notice."
The Minister of Statistics, Maurice Williamson, yesterday encouraged people to use the online option.
"It's quick, easy and secure."
Ms Carter said more than 500,000 forms had already been filed online by yesterday afternoon, and Statistics NZ did not expect all the rest today.
"While there is a Census day, and we want as many people as possible to do it on Census day, there will be a few people who, for one reason or another, do it later," she said.
About 30 per cent of Australians filed their Census returns online in 2011. The proportion was only 16 per cent in Britain in the same year, but 54 per cent in Canada where people were mailed an internet access code instead of a questionnaire.
A Herald Online poll last week found 58 per cent of Herald website readers planned to file their returns online. Only 31 per cent intended to return paper forms and 11 per cent said they would not file returns at all.
The $72 million Census is New Zealand's first for seven years after the planned 2011 Census was cancelled because of the Christchurch earthquake. The questions are identical to the last Census in 2006 except for minor wording changes such as replacing Scotland with India in the options listed for birthplace.
Filling in the forms is compulsory for everyone in New Zealand today, with a fine of up to $500 for non-filers. In 2006 70 people were prosecuted for not filing returns. Despite this, the non-response rate has risen from 4.4 per cent in 1996 to 5 per cent in 2001 and 5.2 per cent in 2006.
The final net under-count was estimated at 1.6 per cent in 1996, 2.2 per cent in 2001 and 2 per cent in 2006.
But demographer Ian Pool said this was still "very, very low" compared with most countries and the Census was still the only source of information on detailed subsets of the population.
He said New Zealand was too small to follow some European countries that have replaced their censuses with sample surveys.
"In South Africa, a 10 per cent sample gives you five million people. In New Zealand it gives you 400,000."
'2016 Census will put us in sync'
Demographer Ian Pool wants us to fill in Census forms again in three years so we can get back into sync with other Commonwealth countries.
The Census that was due in 2011 was postponed until today because of the Christchurch earthquake, and Statistics NZ says the next one will be in 2018. But Professor Pool, an emeritus professor at Waikato University, says it should be in 2016 instead to coincide with Australia.
Australia's 2011 Census found 483,000 people living in Australia who were born in NZ, plus others born there to Kiwi parents.
"The comparisons are extremely important. There is an obviously critical one for New Zealand with Australia," he said.
Most Commonwealth countries have held censuses a year after the turn of each decade since 1801.
Professor Pool said that if New Zealand moved to a new rotation from 2013, comparisons would be much more difficult.
On the web:
census.govt.nz
