Australia's first attempt to conduct an online census is in disarray after a series of cyber-attacks on the website.
The Australian Bureau of Statistics shut down the site to protect data on Tuesday night after four denial-of-service attacks that came from somewhere overseas, chief statistician David Kalisch said.
"It was an attack," Mr Kalisch told the Australian Broadcasting Corporation. "It was quite clear it was malicious."
The two million Australians who managed to access the site before it was shut down were assured that their private data was secure.
"There has been no attack on the information, it was an attack on the system. The information is secure and safe," Mr Kalisch said.
Australia's privacy commissioner Timothy Pilgrim said he was investigating the cyber-attack "to ensure that no personal information has been compromised".
Australian security officials were attempting to determine the source of the attacks, Mr Kalisch said.
He said "a gap" in the digital defences of the bureau of statistics had been fixed and the site would be reopened on Wednesday.
The census is conducted every five years. The decision to conduct the national survey online and to keep the information for four years before it was destroyed instead of the usual 18 months heightened privacy concerns this year.
Several senators announced that they would risk fines by refusing to include their names and addresses in their census forms. Officials attempted to allay fears by boasting that the bureau of statistics had never been hacked.
The site was shut down after a fourth cyber-attack on Tuesday evening and people who telephoned the bureau of statistics for an explanation were told by a recorded message to call back on Wednesday.
While the census focuses on people's circumstances on August 9, forms started to be accepted a week before that date and will continue to be until September. Traditional paper census forms were provided to householders on request.
Conducting most of the survey online was estimated to save 100 million Australian dollars (£58.4m).
Prime minister Malcolm Turnbull said the Bureau of Statistics shut the site "out of an abundance of caution" and would be reopen it once the bureau, government security agency Australian Signals Directorate and the systems provider IBM were confident that it was safe from cyber-attack.
"I want to assure Australians that the unequivocal advice we have received ... is that their Australian census data is safe, it has not been compromised," Mr Turnbull said.
Alastair MacGibbon, the prime minister's special adviser on cyber-security, said while government agencies routinely weathered such attacks without disruption, the Bureau of Statistics was the victim of a confluence of events.
"There were a series of events that only by lining them up end-on-end led to the unfortunate circumstances of last night," he said.
The opposition blamed underfunding for the bureau for the failure, saying it would reduce responses to the census and compromise the results.
"This has been the worst-run census in Australian history and one of the worst IT debacles Australia has ever seen," opposition MP Andrew Leigh said.