Melbourne's two freeway tunnel closures that caused widespread traffic gridlock can be traced back to a single faulty network switch, toll road operator Transurban has said.
The city's twin Burnley and Domain tunnels had to be closed for more than 12 hours earlier this month, forcing more than 120,000 motorists on to other roads encircling the CBD.
News articles at the time speculated that it might have been the result of hacking, but Transurban has now determined that the cause of the computer breakdown leading to the closures was a failed network switch, and not the work of hackers.
The switch was used to send and receive computer messages with a main control room, but its failure created a cascading series of error messages known as a "broadcast storm" across the entire IT network, freezing up the system, the company said.
The secondary system, which was supposed to come online in the event of a failure, didn't work in this instance, Transurban said.
The faulty switch has since been replaced while the company installs further back-up systems and replaces other network switches.
"The computer problems that led to the closure of the Domain and Burnley tunnels on October 3 are completely unacceptable, and we are determined to ensure that it never happens again," Transurban CEO Scott Charlton said in a statement.
Transurban is continuing to defend the decision to close the tunnels on safety grounds, since the IT failure meant that there were no safety systems online in case of a roadside emergency or collision.
The closures were the worst traffic failure in the toll operator's 13 years of running the CityLink road system.