What happened to the census?

The Australian Bureau of Statistics had to shut down the census website after it was overwhelmed with traffic. Here's how it unfolded.

WHAT WENT WRONG WITH THE CENSUS?

TUESDAY:

* 1008 (AEST) - ABS detects significant increase in traffic for 11 minutes causing a system outage of approximately five minutes.

* 1019 - Traffic subsides without action from ABS or software contractor IBM.

* 1146 - Another increase in traffic consistent with a second denial of service.

* 1150 - ABS and IBM activate denial of service mitigation response plan. Short system outage experienced. ABS decides to maintain a block on all international traffic until midnight.

* 1155 - Incident reported to the Australian Signals Directorate for advice on preventing further incidents and intelligence related threat.

* 1658 - "Modest" increase in traffic defended by ABS network firewalls.

* 1815 - Small-scale denial of service attempted but stopped by standard protections.

* 1930 - "Significant" denial of service, taking a different form than previous ones - detected. At the same stage a large increase in traffic to the website occurred with thousands of Australians logging on to complete census.

* 1945 - ABS shuts down online form to protect system from further incidents.

* 2010 - Census Minister Michael McCormack's office notified of outage. He requests briefing from ABS, which is provided within minutes.

* 2032 - Prime Minister Malcolm Turnbull notified, followed by Treasurer Scott Morrison.

* 2038 - ABS tweets that its websites, including the census, are "experiencing an outage" and it is working to restore the service.

* 2050 - Online form system restored but overload protocols activated to prevent connections until system integrity can be assessed.

* 2259 - System restored but not brought online "as a precaution". ABS tweets the census website won't be restored on Tuesday night. It reassures people they won't be fined for not completing the form that night.

WEDNESDAY

* Mr McCormack fronts media at 1030 providing timeline of events.

* He says: "I will be clear from the outset, 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. ABS census security was not compromised. No data was lost."

HOW MANY FORMS WERE COMPLETED?

* 2.33 million online forms submitted before outage and safely stored by 19.33.

* Over 3 million paper forms with households at the moment.

* At its peak, 150 forms were being submitted per second online. Mr McCormack says this was well within system capacity of 260 forms per second.

WHAT ARE THE DEADLINES FOR SUBMISSION?

* Paper forms are not due until September 18.

* Online submissions are due by September 23.

WHAT IS A DISTRIBUTED DENIAL OF SERVICE ATTACK?

* It's an attempt to make a website unavailable by flooding it with traffic from multiple sources - a bit like a huge crowd of people trying to go through one doorway all at the same time.

* The sheer quantity of requests overwhelms the servers, meaning they can't deal with legitimate users wanting to access the site at the same time.

* Because the attack comes from multiple places, traditional ways of shutting it down like blocking the source don't work.

* IT experts say in cases like the census where many people are trying to access the same site, it can be hard to determine what is legitimate and what is an attack because the high volumes of traffic from multiple, legitimate sources can look the same as a DDoS attack.

Source: AAP