The organisation has blamed attacks by hackers for the disaser, with its head, David Kalisch. saying this morning that four attacks had caused the site to go down.
He said an announcement would be made later today as to when the site would be up again.
An ABS spokesman had boasted pre-census that the site could handle "1,000,000 form submissions every hour. That's twice the capacity we expect to need".
The same spokesman was also quoted as saying on Tuesday morning: "There is plenty of reserve capacity to cope if more than 80 per cent of Australians choose to complete the census online."
But this turned out to be mere bragging in the end. The hashtag #census2016 was the top trending topic on Twitter for most of the evening as frustrated Australians gave vent to their feelings.
With the #census2016 website in meltdown the Govt & ABS must rule out any attempt to fine people for not completing it. What a total farce— Sarah Hanson-Young (@sarahinthesen8) 9 August 2016
It will also be interesting to see what the mainstream media, which to a large extent spouted the government line, will have to say about the census today.
Guy Eilon, a senior director at the security company Forcepoint, told iTWire: "Whilst government bodies are taking steps to address information security concerns, not enough emphasis is put on data protection. Security is no longer about reactively defending data and overall information security needs to take a 360 degree approach that incorporates people, processes and technology across the organisation.
"Particularly with increased public scrutiny in regards to spending, government agencies in Australia are operating under tightened budgets and are hesitant to take the necessary steps to protect citizens, networks and sensitive data. Given the Australian Computer Society has forecasted the cost of fraud and cyber-attacks will reach $70 billion by 2020 addressing security across government should be more of a focus."
Eilon added: "There is no one size fits all approach to cyber security and each department needs to be assessed on a case by case basis. What we do know is that today's threat landscape contains increased use of kill chains and attacks that utilise multiple vectors in a blended attack.
"For those who outsource, this means the importance is no longer just about having 'security in-depth' by having multiple vendors, but instead having a single vendor who can provide intelligent and contextual security to stop threats across the entire kill chain. The benefit of outsourcing is the ability to quickly scale security programmes. However it's critical for government partners need to understand compliance obligations as they impact the delivery of services."
Information security analyst Dan Slattery of Webroot said DDoS attacks were reasonably easy to achieve as hackers could purchase botnet resources and point the distributed power of the compromised systems towards a specific server or website.
"These attacks are designed to disrupt access and bring a service offline. They are not designed to compromise data," he said.
Slattery added that there was speculation that the attack was a protest against the ABS' decision to collect and save personally identifiable information for four years.
"There were worries that there may be a data breach and this information would become public or used for malicious purposes," he said, pointing out that the ABS had reported 14 separate data breaches since 2013.
"DDoS attacks are hard to stop, every server that is connected to the Internet is in some ways vulnerable," Slattery said.
"Government and financial sites are often a prime target of these attacks. The best way to mitigate the effectiveness of a DDoS attack is to plan ahead. It is important to have thorough estimates of the typical load on the servers and potential peak usage. Since the ABS was planning on most households filling out the census on 9 August, they would have planned for the potential of having millions of concurrent users."