The Government has reached a confidential settlement with computer giant IBM for costs after the census website crashed earlier this year, leaving millions of Australians unable to lodge their forms.

Key points: Four denial of service attacks caused a 40-hour outage during the 2016 census

Four denial of service attacks caused a 40-hour outage during the 2016 census Cost could be close to $30 million according to the ABS

Cost could be close to $30 million according to the ABS Government and IBM have not disclosed terms of settlement

The 40-hour outage was caused by four Distributed Denial of Service (DDoS) attacks that have been the subject of a blame game between the Australian Bureau of Statistics (ABS) and contractors for months.

Small Business Minister Michael McCormack said the ABS' lead contractor IBM had apologised to the Government and the Australian public, but would not detail the settlement.

"The Government has reached a commercial-in-confidence settlement with IBM on this matter," he said.

"While there are lessons to be learned, more than 96 per cent of Australian households completed their Census — which is on a par with the 2011 Census and greater than the 2006 Census."

While the settlement figure has not been announced, the ABS has confirmed the cost of the trouble-plagued census could be close to $30 million.

Outage 'serious blow to public confidence': cyber security expert

At a Senate inquiry in October, IBM blamed one of its subcontractors for failing to follow geo-blocking protocol to prevent the DDoS attacks. This has since been denied by the subcontractors.

The settlement was announced alongside the release of a scathing report by the Prime Minister's cyber security advisor, Alistair MacGibbon.

Mr MacGibbon said the census failing was a major setback for one of the Government's most respected agencies.

He called the public backlash "a serious blow to public confidence in the Government's ability to deliver on public expectations".

The Government has accepted all of Mr MacGibbon's recommendations, including calls for the Australian Signals Directorate to play a greater role in Government projects.

Mr MacGibbon said the outage amplified existing concerns about privacy and the security of data held by the ABS.

IBM Australia managing director Kerry Purcell offered an unreserved apology for the company's role in the census earlier this year.

He also told senators that no one had been disciplined or sacked over the incident but indicated he was willing to discuss compensation with the Government.

Labor's shadow assistant treasurer Andrew Leigh said the Government needed to take responsibility for the outage and ensure it was not repeated.

"The Government must also ensure that the ABS is properly resourced to manage cyber security threats and provide accurate, reliable data for the Australian people," he said.

"Malcom Turnbull said that 'heads will roll' over the census, but no-one has been has been held to account and none of the four ministers in three years who had oversight of the 2016 census have taken responsibility for this stuff-up."