An angry Malcolm Turnbull has signalled heads will roll after revealing measures were not in place to repel the sort of cyber-attacks that left the census website in meltdown.
The frustrated prime minister admitted there were "serious failures" with the national survey, which was conducted on an opt-out online basis for the first time by the Australia Bureau of Statistics (ABS).
"It shouldn't have happened. I am not happy about it. None of us are," he told reporters in Canberra on Thursday.
Denial-of-service attacks were inevitable - "as predictable as the rain will fall one day, or the sun will come up".
"Measures that ought to have been in place to prevent these denial-of-service attacks interfering with access to the website were not put in place. That is a fact," Mr Turnbull said.
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The failure of prevention measures was compounded by hardware problems, he said - pointing to "big issues" for the ABS and IT company IBM, which was contracted to carry out the census.
"The ABS has inconvenienced millions of Australians," he said.
Mr Turnbull foreshadowed "very serious consequences", following a review to be undertaken by the government's cyber-security adviser Alistair MacGibbon.
"Which heads roll where and when will be determined once the review is complete," he said.
Mr Turnbull, who had tweeted the census was easy to fill out just before the website was taken down, said he was not made aware of the problems until after 8pm on Tuesday.
He confirmed the attack appeared to have come from the United States - or routed through the US.
The census website should be back online on Thursday, he said.
Opposition Leader Bill Shorten criticised Mr Turnbull for failing to take responsibility for the shambles, especially given he changed the minister responsible for the census three weeks out.
"His failure to take responsibility is a failure of leadership," he told reporters in Melbourne.
Mr Shorten again called for an independent Senate inquiry into what went wrong and why.
