Prime Minister Malcolm Turnbull has reassured Australians their data is safe after the census was shut down due to cyber attacks.
The Australian Bureau of Statistics took down the census website in "an abundance of caution" on Tuesday night, Mr Turnbull told reporters in Sydney.
The site would be restored as soon as intelligence agency Australian Signals Directorate and the ABS and its contractor IBM were satisfied it was safe to do so, he said.
"The site has not been hacked, it has not been interfered with, their data is safe."
Labor wants Michael McCormack, the federal minister responsible for Tuesday night's botched census, to resign.
Mr Turnbull said his cyber security adviser Alastair MacGibbon would lead a "very thorough review" into what happened.
Treasurer Scott Morrison said the ABS had an unblemished record when it came to security of census data and had taken every step on Tuesday to protect that record.
"The issue the ABS has been criticised for is being over-cautious last night," Mr Morrison said.
"They chose to place people's data security and any possible, possible compromise to that as being more important than convenience last night."
The Australian Bureau of Statistics is now working to restore its online census facility after it was voluntarily shut down following a series of "distributed denial of service", or DDOS, attacks.
DDOS attacks are not designed to steal data but to frustrate systems - they can be likened to parking a truck across a driveway.
"At no stage during these incidents was any information obtained nor was there any entry into the system," Mr McCormack told reporters in Canberra.
An initial DDOS was detected and investigated just after 10am on Tuesday and was followed by a second incident just before noon.
"It was an attempt to frustrate the collection of Bureau of Statistics census data."
The ABS then decided, on advice from security agencies, to block all international data traffic, which stopped the attack.
But a third incident occurred just before 5pm, followed by another "small scale" DDOS at 6.16pm and a "significant new DDOS" at 7.30pm - when millions of Australians logged on after dinner, adding to the stress on the system.
The ABS to shut down the system to protect 2.33 million forms submitted before the event.
Defence intelligence arm the Australian Signals Directorate is investigating the source of the DDOS.
"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," Mr McCormack stressed.
'It was an attack'
Earlier, the Australian Bureau of Statistics said the census website was shut down after being attacked by foreign hackers.
"It was an attack," chief statistician David Kalisch told ABC radio on Wednesday.
"It was quite clear it was malicious."
The ABS said it was working with the Australian Signals Directorate to determine the source of the attack.
Mr Kalisch said so far it had been very difficult to work out where it came from.
The Australian Privacy Commissioner has also opened an investigation into the ABS and its handling of the attack.
"My first priority is to ensure that no personal information has been compromised as a result of these attacks," commissioner Timothy Pilgrim said in a statement.
Independent senator Nick Xenophon, who had refused to include his name in the census form, questioned how the public could trust the ABS.
While accepting there was no suggestion people's personal data had been hacked, it was still a "major security failure".
The ASD also has questions to answer because it would have certified the system put in place by the ABS for the census.
Senator Xenophon plans wants a parliamentary inquiry into the census when sittings resume in Canberra on August 30.
Cabinet minister Christopher Pyne, who is responsible for the ASD, said it was "just not right" to characterise the census as a failure and dismissed claims the intruders were Chinese.
"Let's not have an orgy of anti-Chinese xenophobia because of claims being made of this or that," Mr Pyne told reporters in Cairns.
In the past Australian government websites have been attacked by Chinese hackers.
Mr Kalisch said he believed the details of people - including Prime Minister Malcolm Turnbull - who had managed to successfully access the site were secure.
"I can certainly reassure Australians the data they provided is safe," he said.
The ABS released a statement around 11.30pm on Tuesday advising the websites was unavailable.
It's reassured Australians they won't be fined for not completing the census on Tuesday.
Mr Kalisch said he expected the site would be back on line around 9am on Wednesday.
Labor calls for census minister to resign
Labor wants the minister responsible for the botched census, Michael McCormack, to resign and believes a re-run is not out of the question.
Shadow assistant treasurer Andrew Leigh is furious the government failed to properly explain its decision to take the national survey online, where it was then subject to foreign cyber attacks on Tuesday night.
"If you can't get the census right how can you govern the country," he told ABC radio on Wednesday.
Dr Leigh criticised the coalition for leaving the position of chief statistician vacant for a year and feared it would now blame the bureaucrats.
"What I'm concerned about is that the government is going to pass the buck," he said.
Dr Leigh urged people to hop online as soon as the website is working again.
Crashing halt
Australia's first online census came to a crashing halt after the Australian Bureau of Statistics website went down completely on Tuesday night.
Many people began experiencing difficulties in the late afternoon, despite more than 1.3 million having successfully completed the Australian Bureau of Statistics' online form, until the site crashed completely around 7.30pm.
Those experiencing error messages and other issues with the website took to social media to complain.
One Facebook user said "is it just me or is the census offline?" while others vented their frustration using the hashtag #Censusfail on Facebook and Twitter.
"The #census2016 site is down. A quick census of the people in my house revealed no one was surprised to hear this," Facebook user Tom Taylor said.
The ABS responded by advising callers to wait until Wednesday to contact the hotline "when (they) expect calls to reduce", assuring people they will not be fined if they do not complete their census on Tuesday night.
The 2016 census has been fraught with contention after it was discovered the ABS would be holding onto data for four years instead of the standard 18 months.
An error message some Australians received. Source: Supplied
Former NSW deputy privacy commissioner Anna Johnston is refusing to fill in the census at all, despite census head Duncan Young repeatedly promising that everyone's data will remain top-secret.
"Hand on heart, the security set-up in order for people to submit their information - it's encrypted all the way through from their browsers into the ABS's internal environment," he said.
"Then we go through the process of separation. The information is isolated so people who can access names can't access the rest."
While Australians have until mid-September to complete the census without incurring a fine, the ABS said people were still encouraged to fill out the form as soon as they could.
About 1.3 million people successfully completed the form until the outage at around 7.30pm (AEST) on Tuesday.
