The minister responsible for the census has directly contradicted the Australian Bureau of Statistics (ABS) by saying its website was not attacked on census night, despite confirming the site was shut down after repeated denial of service attempts.

Key points: Both ABS statistician David Kalisch and the census's social media accounts cited "attacks"

Both ABS statistician David Kalisch and the census's social media accounts cited "attacks" Mr McCormack says four denial of service attempts were made throughout the day

Mr McCormack says four denial of service attempts were made throughout the day Malcolm Turnbull says decision to take site down done "out of an abundance of caution"

Thousands of Australians were prevented from taking part in the census on Tuesday night after the site was pulled down due to numerous attempted denial of service attacks, which overload a website by simulating lots of users trying to access the site at the same time.

No explanation was given at the time, but ABS statistician David Kalisch and census social media accounts later cited a series of overseas cyber "attacks".

Senior cabinet minister Christopher Pyne also cited "overseas hackers" when addressing media on Wednesday morning — language that was not backed by Michael McCormack.

When pressed on his language, Mr McCormack said he felt that "by saying attacked, it looks as though and it seems as though and it is so that information was then gained".

He told reporters at Parliament House that four denial of service attempts were made throughout the day, leading to the census website being taken offline on Tuesday evening as a precaution.

"A denial of service is an attempt to block people from accessing a website," he said.

"No census data was compromised and no data was lost."

Sorry, this video has expired Minister says census online form outage was not an attack or hack

Mr McCormack said he had been assured the system was able to handle increased traffic from people attempting to fill in the census, something questioned by an internet expert this morning.

Mark Gregory from RMIT University said the site could have been brought down simply by increased traffic.

"Really if you had five or six million households all trying to do the census at the same time, that's very similar to a denial of service of attack," he told the ABC.

"So we need some evidence, some proof that this was from outside Australia and not just simply Australians trying to do the census.

Special cyber security advisor to the Prime Minister Alastair MacGibbon joined Mr McCormack at Parliament House and cited the failure of a geo-blocking service as part of last night's disruptions.

Mr MacGibbon said it was "not abnormal" for Australian Government services to be subject to denial of service attempts, but cited the failed service as one of the main defences against such attempts.

He could not name the source country of the attempts, but said it was being investigated.

ABS retains 'unblemished' record on census data security

Sorry, this video has expired PM says ABS retains 'unblemished' record on census data security

Loading

Prime Minister Malcolm Turnbull said he shared the ABS's "regret" over the inconvenience caused by the site being taken offline, but stressed Australians' data was secure.

Mr Turnbull said the decision to take the site down was done "out of an abundance of caution".

"The site will be restored as soon as the Australian Signal Directorate and the ABS and IBM are satisfied that it can be restored with all of the necessary defences against denial of service and other attacks are in place," he said.

"And the public will be advised as soon as that is done."

The ABS said the census website remained offline on Wednesday night.

Mr Turnbull said there would be a review into Tuesday night's events but maintained the ABS retained its "unblemished record" in regards to the security of census data.

"The one thing that is absolutely crystal clear is that there was no penetration of the ABS website," he said.

"This is based on the advice we have."

Opposition Leader Bill Shorten has called for a Senate inquiry into the events, describing the Government's handling of the census as "gold class incompetence".

Michael McCormack (left) contradicted the ABS's David Kalisch, saying there was no attack on the website. ( ABC News )

ABS has had five years to get this right: Xenophon

Crossbench senator Nick Xenophon is also planning to push for a Senate inquiry, after he announced earlier this week he would not put his name on the form, citing privacy concerns.

Greens senators Scott Ludlam, Janet Rice, Sarah Hanson-Young, Lee Rhiannon and Larissa Waters also said they would not be providing their names, though none predicted a denial of service attempt.

Senator Xenophon was accused of "tinfoil hat" politics by Coalition frontbencher Christopher Pyne over his concerns.

He told reporters in Adelaide the census had been a "debacle".

"For the Government, for the ABS to say this is not a cyber attack is a bit like Monty Python's black knight saying that they just suffered a flesh wound," he said.

"I will be proposing a Senate inquiry, an urgent Senate inquiry into this debacle as soon as the Senate sits."

Labor frontbencher Andrew Leigh said the Coalition should not "pass the buck" for the census to the ABS.

Dr Leigh told the ABC it was a Government decision to move to online forms for the national survey and, as a result of the disruption, the reliability of the data was questionable.