The Australian Bureau of Statistics said its sites were hacked four times yesterday by groups overseas, which may have caused the site to crash during Tuesday's Census.
Chief Statistician David Kalisch told ABC radio the four attacks were a deliberate attempt to sabotage the site.
"The scale of the attack, it was quite clear it was malicious," he said.
"It was an attack, and we believe from overseas.
"Steps have been taken during the night to remedy these issues and I can certainly reassure Australians that the data they provided is safe."
The Census website remains down on Wednesday morning, after millions of Australians sought to complete the compulsory form on Tuesday night.
The much-vaunted eCensus had been tested to withstand 150 percent of the anticipated traffic.
Something clearly went wrong.
By 8pm, the Census website appeared to be completely down.
By 10pm, the website was partially back online, but was sending users error messages, saying the website was taking too long to respond or that the "system is very busy at the moment" and participants should "try again in 15 minutes".
By 11pm, the ABS admitted defeat and confirmed to the public via Twitter that the website would not be restored that night.
Shortly after midnight the Minister for Small Business Michael McCormack - the minister responsible for the Census -- issued a statement confirming the outage. He reassured Australians that have until September 23 to complete the Census before facing fines.
McCormack did not explain the reasons for the outage or when the Census website might be back online.
"I have been in constant communication throughout the night with Australian Statistician, David Kalisch, in relation to this matter," the statement read.
"I am informed by the ABS that a thorough process will be undertaken to ensure all households are counted as part of the Census."
By 6.30am, Kalisch said the site had been subject to a denial of service attack.
While the eCensus software is managed in-house, the ABS chose to outsource the eCensus hosting to IBM Australia, at a cost of $9.6m. Melbourne-based firm Revolution IT was awarded hundreds of thousands of dollars this year to perform 'load testing', to determine the system's resiliency.
When the IBM contract was awarded, the ABS said:
"Due to the peak volume of the online form during Census 2016 it was decided that contracting IBM would provide the best value for money and management of operational risk."
That is, the ABS would not have to build its own infrastructure to cater for the Census night spike, and could rely on IBM's Baulkham Hills data centre.
And that traffic was expected to spike. While roughly a third of Australians completed the last Census online, this number was expected to jump to almost 70 percent this time around. (We would link to the ABS statement on this, but, er, the site's still down).
The Sydney Morning Herald reported an estimated 16 million Australians were to have sought the eCensus page on Census day, but experts suggest that modelling may not have account for the after-dinner spike.
Canberra-based software expert Paul Brebner told the Herald the ABS was prepared for 1 million people to be online at once, but his own numbers suggested it needed to be ready for 3 million simultaneous users between 6pm and midnight.
"You can't assume a uniform load across the 24 hours," he said. "If they were expecting 16 million people over 24 hours, that would be okay, but that's not how loads on websites work at all. There's often one very big spike, and it's hard to see how long it will stay in that level."
As to the Australians who had been hoping to complete the Census on Census Day itself, the ABS has said users will not be fined for completing it late.
Many still vented their frustrations on social media at the time wasted. Some also expressed renewed concerns about their privacy, given the apparent inadequacies of the ABS' website.
The outage is likely to prove another political headache for Malcolm Turnbull, after the Australian Bureau of Statistics has spent weeks defending claims that the Census endangered participants' privacy.
Some Labor MPs have already taken to Twitter to criticise the Government's implementation of the first all-online Census.
Not long before the outage, Liberal MPs -- including the Prime Minister -- had been touting the importance and ease of filling in the Census.
Head of the 2016 Census Program Duncan Young told The Huffington Post Australia on Monday the website developers were prepared for an onslaught of Australians logging on, and a website crash would be unlikely.
The online Census is meant to be quicker to fill out, taking about 26 minutes on average to fill out while the paper form takes more than 30 minutes on average.
According to Fairfax, a Census spokesman earlier on Tuesday evening 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."
However, we highly doubt likes of South Australian Senator Nick Xenophon and Greens Senator Scott Ludlam would be bothered by the crash. The politicians have boycotted the Census, refusing to list their names on the national survey -- after it was announced names would be kept on the system for four years instead of 18 months.
This however, is not part of the statistics provided on the ABS website. But you can read all about that, and other questions you may have regarding the Census here.