By Phil Mercer
BBC correspondent in Sydney
Australia's largest ever counter-terrorism exercise is under way.
A series of exercises will be held this year
There will be five days of simulated attacks to test the preparedness of the government, the security forces and the emergency services.
The exercise - codenamed Mercury 04 - is the first of five anti-terrorism drills planned this year.
It comes as the Australian government reviews domestic security in the wake of the bomb attacks in Spain last week, which killed 202 people.
The hijacking of an oil rig off the cost of Darwin by masked gunmen is among the first challenges faced by Australia's counter-terrorism teams.
Details of where and when these simulated attacks will take place are known to just a handful of people.
For the first time, drills will be held simultaneously across Australia in four states and territories.
They will involve the prime minister, senior cabinet figures, local political leaders and more than 3,000 other personnel.
New chemical, biological and radiological equipment will be tested.
Plans to protect the supplies of water and electricity in the event of an attack will also be scrutinised, as will the ability of hospitals to cope with mass casualties.
Mercury 04 has been planned for the past year and a half.
The bombings in Madrid have given the exercise a greater urgency and relevance.
A senior FBI agent warned in Sydney last week that it was inevitable Australia would at some point be attacked by extremists.
Over the weekend the federal government said a review of maritime security in Australia would be carried out.
Senior ministers warned that a hijacked freighter could be turned into a floating bomb to target iconic landmarks, such as the Sydney Opera House.