The census website has been restored more than 40 hours after being taken down due to cyber attacks and a hardware failure.
The online form became available on Thursday afternoon, after the Australian Signals Directorate gave the all-clear.
Earlier, an angry Malcolm Turnbull 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.
A clearly 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 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 inevitably were going to happen to the census website, especially given its high profile.
"A denial-of-service attack is as predictable as the rain will fall one day, or the sun will come up," Mr Turnbull said.
"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."
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.
Mr Turnbull, who had tweeted the census was very 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 be coming from the United States - but it doesn't mean those involved were American, because it is possible to route traffic through the US.
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 called for an independent Senate inquiry into what went wrong and why.
The government is now hoping millions of Australians will complete their forms in order to provide enough information for a statistically useful census.
