The Australian Bureau of Statistics (ABS) says the problems that emerged with its online services yesterday were caused by international denial-of-service attacks.

The ABS has been facing criticism since early on August 9, because of the repeated failure of its IBM SoftLayer-hosted online census. IBM has been putting the system together since winning the nearly AU$10 million contract in 2014.

David Kalisch told ABC Radio there were four attacks: the last, at 7:30 pm, triggered the decision to pull the service.

"The online census form was subject to four denial of service attacks yesterday," he said.

“The first three caused minor disruption, but more than two million forms were successfully submitted and safely stored,” Kalisch said.

He added that the Australian Signals Directorate, the nation's sigint agency, has been called in, and while Kalisch's statement said the attack came from overseas the ASD said it's “very difficult” to locate the source of an attack.

After pulling the system offline completely around 7:30 pm on August 9, the bureau said at 10:59 pm it would not be restored.

ABS & Census website are unavailable. The service won't be restored tonight. We will update you in AM. We apologise for the inconvenience. — Census Australia (@ABSCensus) August 9, 2016

There will be no fines for completing the Census after Aug 9. There’s still plenty of time to complete the Census. Thanks for your patience. — Census Australia (@ABSCensus) August 9, 2016

The collapse of the service raises serious questions not just about the ABS' ability to handle the load, but the accuracy of the data it will collect, with many complaints on social media that users don't know how much of their data was saved before outages.

The process was already contentious, with the bureau facing a couple of weeks of criticism over its decision to hold individuals' names for four years.

Since one of the reasons given for taking the system offline was to “protect the integrity of the data”, the bureau will need to explain how well it can recover the incomplete forms.

The ABS had spent hundreds of thousands on load-testing the service, and in the lead-up to Census night it was confident the system would cope with the load, claiming the site could handle a million logins per hour.

The Register has already asked the office of the Prime Minister, Malcolm Turnbull, for his view on the cloud-hosted collapse. ®