Millions of Australians were prevented from taking part in the national survey on Tuesday night.
The Australian Bureau of Statistics (ABS) had boasted only hours before that its website would not crash.
Debate about privacy concerns has been raised despite assurances from the government that security would not be compromised. The prime minister Malcolm Turnbull assured the public that their personal information was not compromised.
Every five years, everyone in Australia is required to fill out forms are compiled to provide a snapshot of the country.
Two-thirds of Australians were expected to complete the census online this year, rather than on paper.
Chief statistician David Kalisch said the Australian Signals Directorate are investigating, but they did note that it was very difficult to source the attack.
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