Australia's bean counters and cyber detectives hope to avoid the embarrassment of another bungled census.
Some 3.3 million surveys have already been returned, which covers about 8.5 million Australians.
Assistant Treasurer Michael Sukkar said there was no guarantee cyber attacks wouldn't impact the census, with some people always wanting to test government systems.
"Thus far we've been very happy with the security measures put in place," he told Sky News on Tuesday.
"We'll be hoping that for the remaining two-thirds of Australians who are hopefully going to lodge their surveys over the course of today and this evening that it will all be smooth sailing."
After trusting tech giant IBM with crucial infrastructure last time, which spectacularly crashed, the Australian Bureau of Statistics has rebuilt the census system with consultancy firm PwC Australia and Amazon Web Services.
Checked and rechecked by federal experts in the lead-up, amid a rise in successful ransomware attacks on Australian businesses over the past six months, it is hoped any cyber attacks will fail.
Australian Statistician David Gruen says stress on the census system has been eased by Australians completing the survey early.
Mr Sukkar said the Australian Cyber Security Centre, the Department of Defence and the Digital Transformation Agency have ensured "state of the art" cyber security protections are in place.
"We're very confident about not just the robust security but the protection of people's data," he said.
Amazon Web Services has been certified by federal cyber experts for storing and processing highly sensitive data rated at the "protected" security classification level.
The data is expected to give an unusual snapshot compared to other years because of COVID-19 lockdowns and border restrictions.
Dr Gruen is predicting the survey will show a mixture of pandemic-specific information and longer-term trends.
He believes the trend of people moving out of capital cities, perhaps accelerated during the pandemic, will be documented in this year's census.
That would give policymakers a clearer idea of future needs for transport, health, education and other services.
Failure to fill out the compulsory form will attract a polite reminder and then a fine.
Australian Associated Press