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Australia's first online national census was in chaos on Wednesday after the survey website crashed overnight due to a possible cyber attack, raising concerns over the country's cyber security and criticism of its slow internet services.

"It was an attack and we believe from overseas," Australia’s chief statistician, David Kalisch told Australian Broadcasting Corp radio.

REUTERS/Dado Ruvic

Kalisch said that no data from the 2.3 million forms already submitted to the Australian Bureau of Statistics (ABS) had been stolen. "We have it at the ABS. No one else has it," he said.

The census provides a snapshot every five years of the living conditions of Australia's 24 million people, detailing incomes, religious and ethnic backgrounds, marital status, etc.

The minister responsible for the survey, Michael McCormack, refused to call the online crash an attack, but rather a "denial of service attempt" when the website was deliberately overloaded.

He said the site was equipped to handle heavy traffic, but there was a spike in visitors so steep that a router overloaded and the website was closed as a precaution.

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"This was not an attack, nor was it a hack, but rather it was an attempt to frustrate the collection of Bureau of Statistics census data," McCormack said.

As authorities scrambled to provide a cohesive explanation for why the census was not completed for the first time in its 105-year history, some politicians and privacy advocates said the incident vindicated their security concerns.

Australia's internet services rank 48th in the world, by average speed.

Some independent Senators boycotted the census because for the first time it was mandatory for Australians to identify themselves in the survey.

"It shows woeful disregard for Australian people's privacy and data," Anna Johnston, a privacy lawyer and director of consultancy Salinger Privacy, told Australian Associated Press.

The failure has also led to criticism of Prime Minister Malcolm Turnbull's decision to scale back a A$38 billion ($29.2 billion) upgrade of Australia's internet infrastructure, stopping short of connecting homes directly to a broadband network amid cost overruns.

Australia's internet services rank 48th in the world, by average speed, according to the most recent State of the internet report by Akamai Technologies, an IT company specializing in internet speed technology.

Both Australia's Signals Directorate, an intelligence agency, and the government-appointed privacy commissioner, Timothy Pilgrim, are investigating the crash.

"My first priority is to ensure that no personal information has been compromised as a result of these attacks," Pilgrim said in a statement.