The Australian Bureau of Statistics (ABS) is this week sending final reminder notices to dwellings from which it has not had a response.

As of 8am on Monday, the ABS estimates it has received 9,105,131 completed forms in the 2021 census, almost three weeks after the official night.

The tally is based on 7,381,157 forms submitted online through the census digital service and 1,723,974 paper forms mailed to the ABS by households.

The ABS's estimate for forms received in NSW is 2,836,691, about 83 per cent.

The Census of Population and Housing is compulsory and counts each house and person in Australia on census night.

If a house or property was vacant on census night, the ABS asked people to let it know by reporting it online.

"It's important that we get an accurate picture of the number of unoccupied or vacant properties because the census counts all houses and properties across Australia in addition to counting people," census executive director and national spokesman Andrew Henderson said.

"This includes second homes, investment properties and holiday homes.

"This helps inform and improve service delivery, urban planning and local infrastructure in your community.

"Our census field staff are currently visiting communities to remind those who haven't completed to do so today.

"For those with completed paper forms, remember to mail the form to the ABS in the reply-paid envelope.

"In delivering reminders, the ABS is working in close collaboration with local authorities to ensure census staff are operating in line with the relevant COVID-19 restrictions."

People can complete now without a census letter or form by going online and selecting the 'get a census number' option.

They can also request a paper form online, or by calling 1800 512 441.

For people who need assistance completing their census, there are a range of help and support options available at www.census.abs.gov.au.

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