CANBERRA - The global reach of violent Islamic extremism and the power of jihadist ideology have been hammered home to Australia by the arrests yesterday of men allegedly plotting suicide attacks against military bases in Sydney and Victoria.

The group allegedly intended to rush the bases with automatic weapons and kill indiscriminately until they were themselves killed.

Federal Police Acting Commissioner Tony Negus said yesterday's early-morning raids across Melbourne's northern suburbs "disrupted an alleged terrorist act that could have claimed many lives".

One man, 25-year-old Nayes El Sayed, has been charged with committing an act in preparation for a terrorist act, police have been given more time to question another man, and three others have so far been detained.

The men are believed to be linked to the Somali-based al-Shabaab terror group that is waging a bloody war against the African nation's interim Government, and which has links to the international al Qaeda network.

Although primarily engaged in its national campaign, al-Shabaab regards itself as active in the global war against "enemies of Islam" and recruits and trains foreign fighters from countries including Australia, Britain and the United States.

Australian citizens are known to have trained in Somalia, one is believed to have been killed there, and the plot revealed after the mass raids across Melbourne's northern suburbs before dawn yesterday appears to have been fomented by men frustrated in their attempts to join al-Shabaab.

"It shows you what's happening in the shadows that we don't know about," Dr Leanne Piggott, of Sydney University's Centre for International Studies, said.

Prime Minister Kevin Rudd, who was briefed about the impending raids on Sunday, said the existence of another alleged terror cell with plans to attack within Australia showed the nation continued to face an enduring threat from terrorism.

"The sobering element to emerge from today's development is the reminder to all Australians that the threat of terrorism is alive and well [which] requires continued vigilance on the part of our security authorities, and we propose to maintain that vigilance into the future," he said.

Counter-terrorism agencies have warned that Australia faces a growing risk of home-grown terrorism rising out of young men influenced by the ideology of global jihad and the teachings of local extremists.

Terror expert Professor Clive Williams of Macquarie University said: "It has become more difficult for people to get into Somalia, particularly if they are young people who fit the profile of joining al-Shabaab. That perhaps has encouraged people who can't get there to do something at home, which is what al Qaeda has said in the past."

So far 12 people have been convicted of terror-related crimes in Australia - including members of two cells planning major attacks in Sydney and Melbourne - and more than 10 others are before the courts.