People trying to complete the census online have been met with numerous error messages, told the system is "overloaded", and advised to call back in two days when they phone a hotline for information.
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By 8pm on Tuesday evening, it appeared the website abs.gov.au had crashed entirely, as an estimated 16 million people were due to log on to complete the census.
According to multiple websites checked by Fairfax Media, the Bureau of Statistics servers had stopped responding, with one advising: "abs.gov.au is DOWN for everyone. It is not just you."
The troubles began at around 5pm, when people trying to access the form were stopped by messages including a "code 31" error, which said the request "could not be completed because a problem was encountered".
Some people were able to get through, but the errors appeared to escalate through the evening until the website stopped responding.
Others said they had tried to log in on multiple devices and still had no luck getting through, with one person claiming they had tried for two hours before they eventually gave up.
However, some people who managed to fill in their details still faced errors, with messages popping up around 7.30pm that the census could not be submitted as "a problem was encountered".
Earlier, the Bureau of Statistics advised anyone who had an error message to wait for around 20 minutes before logging back on.
On Tuesday evening, a Census spokesman said the online system was operating "as expected".
"We have currently received more than 1.3 million successful online submissions," the spokesman said.
"The system is operating smoothly and as expected. If anyone has experienced a local technical issue, they can try again on another device or contact the Census Inquiry Service on 1300 214 531 if they continue to have technical issues, or go to the Troubleshooting page of the Census website."
The Census Inquiry Service was deluged with calls on Tuesday evening, with Census advising anyone with issues to try calling back after August 10.
Earlier, 2016 census manager Duncan Young said census day was going "very well" and online forms would save more than $100 million for taxpayers.
"The online form is fast, it's very easy and it's completely secure and Australians are enjoying the experience of filling out their census online," Mr Young said.
"The form has the capacity it needs to scale up tonight for the large volumes of response that we are expecting."
