The 2016 national survey was dogged by an online debacle that saw millions of people locked out of the website.

Australian Statistician David Gruen said there had been a huge response to offering people the chance to complete the census ahead of Tuesday.

"We've done it for two reasons - one to reduce the load on census night but the second reason is for convenience," he told ABC radio on Monday.

With millions of Australians in lockdown on census night, the data is expected to give an unusual snapshot compared to other years.

Dr Gruen is predicting the survey will show a mixture of pandemic-specific information and longer-term trends.

"Clearly there are going to be very big differences depending on whether you're living in one of the locked-down places or living in perhaps Perth or Adelaide," he said.

He believes the trend of people moving out of capital cities will be reflected in this year's census.

"That's had probably something to do with the effects of the pandemic."

Questions about whether people are defence force veterans will be included for the first time to give higher quality information about serving and former personnel.

Equality Australia chief executive Anna Brown has criticised the government for not asking about sexual orientation or gender identity.

"Once again, lesbian, gay, transgender, intersex and queer people won't be properly counted in this year's census," she said.

"It's a missed opportunity to inform crucial decisions about what services are provided to our communities, and where."

Dr Gruen said the Australian Bureau of Statistics considered including those questions but decided they were better asked by trained interviewers.

"There are currently two big surveys in the field collecting information on gender identity and sexual orientation."

Australian Associated Press