The Australian Census 2016 has been thrown into disarray by a cyber-attack which shut down the Australian Bureau of Statistics website just as millions of people were sitting down to complete their online details.

Head statistician David Kalisch said the ABS had been a victim of a distributed denial of service, or DDOS. Such attacks are designed to cripple target websites by flooding them with useless internet traffic so that requests from legitimate users cannot be serviced, causing the “denial” of service.

DDOS attacks are generally launched by cyber-criminals using “botnets”, also known as “zombie armies”.

These are groups of computers infected with malware that are remotely controlled by cyber-criminals, generally without the owners’ knowledge. They are sold or rented out every day over the ‘dark web’, a vast encrypted network where criminals do business online.

Botnets change hands here for just a few dollars, and can be used to attack a website from multiple points at once in a massive synchronised hit.