The US Department of Homeland Security's Cyberstorm exercise tested procedures, communication channels and responses in the event of an attack, as well as international communications protocols between countries.
GovCERT.au, the Attorney-General's security unit, coordinated the tests in Australia. A number of other government departments were also involved, as well as counter-terrorism and policing experts, and AusCERT.
Attorney-General Philip Ruddock said such exercises were the only way to effectively test systems against theoretical attacks.
"Terrorists are constantly seeking new and innovative ways to attack and disrupt our way of life," said Ruddock.
"By conducting exercises such as these we increase Australia's ability to detect, prevent and respond to cyber attacks."
The tests form part of a week-long exercise by the US Department of Homeland Security that will culminate in attempts to exploit vulnerabilities and cause havoc by disrupting transport and communications systems.