Australia's census website was shut down after an alleged attack by foreign hackers, an embarrassing incident that forced the federal government yesterday to assure citizens their personal information was safe.
Following weeks of controversy over fears that the online census posed a privacy risk, officials finally went live with the census website on Tuesday but took it down at 7.45pm after four separate "malicious" attacks.
The Australian Bureau of Statistics (ABS), the government agency overseeing the census, said it is believed foreign hackers had deliberately targeted the website.
"It was an attack, and we believe from overseas," the bureau's head, Mr David Kalisch, told ABC Radio.
"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."
The attempted attacks - and the possible source nation - are being investigated by the Australian Signals Directorate, the military's cyber security intelligence agency.
Officials said there were "four denial of service attacks", which typically involve an attempt to overwhelm a website with a deluge of simulated users.
The census is held every five years and it is compulsory for everyone in the country to take part, including foreign visitors. It includes questions on topics such as work and marital status, place of birth, languages spoken and religion.
But this year's census marked the first time the government has encouraged people to complete the confidential survey online, leaving many concerned about privacy. Previously, it was submitted mainly using paper forms.
Prime Minister Malcolm Turnbull yesterday assured Australians that "their data is safe".
He has ordered a review but said the website was shut "out of an abundance of caution", following an attempted denial of service which caused a hardware failure and overloaded a router. "The site has not been hacked; it has not been interfered with," he said.
Still, Australia's Privacy Commissioner Timothy Pilgrim said he will review the attacks to ensure no personal information was compromised.
The opposition warned that the shutdown of the website could affect public confidence in the census. "It is humiliating when the government asks millions of Australians to fill out the census and the government can't even get that right," said opposition leader Bill Shorten.
Cyber security experts offered a range of explanations for the attacks.
Some said Chinese hackers may have exacted retribution against Australia over a feud in recent days between two rival Chinese and Australian Olympic swimmers.
Others said the website may not have been hacked and that it may simply have suffered from a rush by citizens to complete their census forms after returning home from work. "Really, if you had five or six million households all trying to do the census at the same time, that's very similar to a denial of service attack," Dr Mark Gregory of RMIT University told ABC News.
"So we need some evidence, some proof that this was from outside Australia and not just simply Australians trying to do the census."
About two-thirds of Australia's 24 million population are expected to complete the census online this year. ABS said about 2.33 million online forms were completed and safely stored before the shutdown.
Australians have until Sept 23 to complete the census.
But the government said the website will not be restored until it has been cleared by the military's cyber security agency.