Census chiefs were wrongly confident about security and made little preparation against cyber attacks last August, when millions of Australians were blocked from completing the online survey.

The site became overloaded and was shut down for 40 hours after a Distributed Denial of Service (DDoS) attack simulating lots of users trying to access it at the same time.

A Freedom of Information (FOI) investigation sought documents relating to DDoS in the three months before census night from Australian Bureau of Statistics (ABS) boss David Kalisch and census manager Duncan Young.

In response, the ABC received three heavily-redacted pages sent to Mr Young by a colleague.

Just nine lines were about the DDoS threat.

"At high level, our architecture resists DDoS attacks via use of multiple layers of security," the April email said.

"In general terms, our experience is that most external DDoS attacks are stopped at the border firewall or, increasingly, in front of it, as cloud services are more widely utilised."

The ABS said it conducted a wide-ranging search for documents.

But it uncovered no further correspondence, including nothing about DDoS to or from David Kalisch — Australia's chief statistician — in the final months before August 9.

Politicians take responsibility

Labor MP Andrew Leigh said the ABS should have been asked if it was prepared for a DDoS attack. ( ABC News: Nick Haggarty )

Federal Labor said the documents — or lack thereof — were further proof of the Turnbull Government bungling the event.

"The census is the biggest peacetime logistical operation in Australia," shadow assistant treasurer Andrew Leigh said.

"And yet the Turnbull Government weren't prepared for the inevitable denial of service attacks that ended up shutting it down."

Dr Leigh, whose Canberra electorate includes the ABS' headquarters, would not be drawn on the public servants' performance.

"Politicians ultimately bear responsibility," he said.

"You need to have a minister who is asking the hard questions, and the right question to ask in Australia's first online census is: 'Are you prepared for a DDoS attack?'"

Census Minister Michael McCormack (left) admitted improvements were needed, but emphasised the survey's successes. ( ABC News )

Small Business Minister Michael McCormack oversees the ABS and said improvements were needed.

But he pointed to the census' successes.

"More than 96 per cent of Australian households completed their census, which is on par with the 2011 census," Mr McCormack said.

"A record 58 per cent of Australians completed their census online."

Mr McCormack also emphasised his hands-on approach ahead of the next nationwide survey.

"I meet regularly with the Australian Bureau of Statistics, as planning gets underway for the 2021 census," he said.

The minister said the Government had accepted all recommendations of a census review by the Prime Minister's cyber security advisor Alastair MacGibbon.

ABS slammed for not challenging IBM

The census website was pulled down for 40 hours after a fourth DDoS traffic spike.

Mr Kalisch last year said he was assured by contractor IBM the system was "robust and was ready to go".

However, Mr MacGibbon's investigation slammed the agency for not independently testing its systems.

"IBM's assurances were taken at face value: if IBM said in an email that DDoS protections worked, the ABS took comfort," the report said.

"The ABS provided minimal challenging or inspection, and did not use third parties to test and verify that DDoS protections were actually in place and effective."

The ABS' contract with IBM explicitly included DDoS protection — something that saw the technology giant pay the Commonwealth millions of dollars in compensation.

Prime Minister Malcolm Turnbull insisted heads would roll over the disaster.

"There will be some very serious consequences for this," Mr Turnbull said last August.

Mr Kalisch reportedly earns more than $700,000 a year.

Mr Young was the ABS' chief of technology infrastructure for nearly five years before running last year's census.

In response to a series of questions, the ABS referred the ABC to the MacGibbon report and a Senate inquiry.

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