This year will be very much a locked-down pandemic census. Mostly online, little physical contact from census workers and a largely, and unfortunately, home-bound audience to fill out the forms. It must be filled out, even in quarantine.

The current lockdowns of Australia's largest population centres, including greater Sydney and Melbourne, will be a major feature of the data being collected, collated and crushed by the Australian Bureau of Statistics.

"This census is perhaps even more important than most because it will give us a snapshot of how Australia is living and functioning right now during the pandemic," Deputy Australian statistician Teresa Dickinson told The Canberra Times.

"And we'll be able to find out lots of information about employment and about how people are moving around Australia. We hear lots of anecdotes and there is some data on the tree change/sea change environment that we're living in.

"This census will give us really detailed information about the way people are moving around the country, how they're working, how industry is changing."

It will be the first for Australia in the middle of a pandemic, but there will be much to compare to the census conducted 100 years ago. The 1921 census came just two years after the devastating Spanish flu of 1918-19.

"This is likely to be a census that's going to be referenced for a very long time," Ms Dickinson said.

"We've got researchers going back now looking at the census in 1921, after the Spanish flu epidemic, and looking to see what society was like then and how it was responding after a pandemic. So we think that this is a very important census for Australia."

Around 2 million households are expected to have completed and submitted census forms in advance of census night. There are expectations around 75 per cent of people will complete the form online.

Census data helps plan and deliver services such as education and health care, so it is important the information is correct and complete. There are concerns and efforts around including people such as the homeless and those in remote regions.

There are new questions on long-term health conditions and defence services, but there are no new questions on sexual orientation and gender identity after it was proposed and nixed late last year.

Census 2021 is also expected to be a high-profile target for cyber attacks, such as the embarrassment of Australia's first online census, the infamous 2016 'censusfail' debacle.

The census website was flooded and overwhelmed on the first day by a number of distributed denial-of-service (DDoS) attacks. It related to a hardware failure by IT contractor IBM. The ABS, concerned about security of the census data, took the website offline.

Ms Dickinson says the ABS has received the best possible state-of-the-art advice on how to conduct a safe and secure census.

She says government and business systems are under attack all the time and the census is being monitored very carefully.

"In terms of what we're experiencing for us, I'd call it business as usual," she said.

"There's a number of ways that people might want to be mischievous with the census. One is through cyber security and the other is through perhaps impersonating our field offices and setting up fake websites that look like the census website. So we're doing a lot of surveillance of all those sorts of things.

"At the moment, we're not seeing anything that causes us concern or alarm."