The Australian census website was shut down by what authorities said was a series of deliberate attacks from overseas hackers.
Millions of Australians were prevented from taking part in the national survey on Tuesday night.
The Australian Bureau of Statistics (ABS) had boasted only hours before that its website would not crash.
The prime minister assured the public that their personal information was not compromised.
Every five years, everyone in Australia is required to fill out forms are compiled to provide a snapshot of the country.
Two-thirds of Australians were expected to complete the census online this year, rather than on paper.
Debate about privacy concerns has been raised despite assurances from the government that security would not be compromised.
'Not an attack'
Prime Minister Malcolm Turnbull said that public's personal information was safe and and stressed the "unblemished record" of the ABS.
"The one thing that is absolutely crystal clear is that there was no penetration of the ABS website," Mr Turnbull said.
"What you saw was the denial of service attack or a denial of service attempt which, as you know, is designed to prevent access to a website as opposed to getting into the server behind it. Some of those defences failed, frankly."
Assistant Treasurer Michael McCormack, who was responsible for overseeing the census, denied that the national survey was "hacked" or "attacked".
"It was an attempt to frustrate the collection of data," he said.
"People should feel rest assured their data is safe."
Calls to resign
The comments contradict earlier comments issued by the ABS which stated that there were four "attacks".
The opposition party called for Mr McCormack to resign over the website crash.
"This has been the worst run census in Australian history," said Andrew Leigh, the assistant shadow treasurer.
"If we don't get an accurate snapshot on census night, we can't allocate resources properly."
The ABS is now working with authorities to determine the source of the "denial of service" attacks.
"The Australian Signals Directorate are investigating, but they did note that it was very difficult to source the attack," chief statistician David Kalisch told the ABC.
"The scale of the attack, it was quite clear it was malicious.
"Steps have been taken during the night to remedy these issues and I can certainly reassure Australians that the data they provided is safe."
In the lead-up to the census, crossbench Senator Nick Xenophon's concerns about privacy were dismissed by the government as "tinfoil hat" politics.
He said it wasn't clear who should be wearing the hat now.
"Look, there are real concerns," Mr Xenophon said.
"The census, the ABS, has had five years to get this right."
After weeks of reminders to "get online August 9", millions of Australians were frustrated to find they could not complete the survey.
Thousands of people poked fun at the situation on social media with references to the popular television shows including The IT Crowd, The Simpsons and Monty Python.