Arrest in Sydney comes on same day as police raid homes across Melbourne.

A teenager has been arrested after Australian police alleged he had been planning to launch a terrorist attack in Sydney.

The 18-year-old's arrest yesterday came on the same day counter-terror raids in Melbourne targeted homes of relatives of the five men charged over a plan to flee Australia to fight with Isis (Islamic State).

The teenager was expected to be charged with planning a terrorist attack, the Australian Federal Police said. They said another charge - preparing to enter a foreign country to engage in hostile activities - was expected to be filed in connection with an earlier attempt by the teen to travel to Syria.

If convicted, he could face a maximum sentence of life in prison.


"The planning was occurring now and we would say that an attack was probably imminent," New South Wales Police Deputy Commissioner Catherine Burn told reporters.

The teen had been making arrangements to get a firearm, Australian Federal Police Acting Deputy Commissioner Neil Gaughan said. Officials believe he was working alone, and his arrest means there is no longer any threat to public safety linked to the alleged plot, Gaughan said.

The teen, who had been on authorities' radar since last year, was stopped at Sydney airport in February when he tried to travel to Syria, Gaughan said. His passport had been cancelled and he was prevented from leaving the country.

Meanwhile, in Melbourne, Islamic preacher Musa Cerantonio, Shayden Thorne, Kadir Kaya, Antonio Granata and Paul Dacre are due to face the Melbourne Magistrates Court tomorrow charged with making preparations for incursions into foreign countries to engage in hostile activities.

Homes linked to the five were raided yesterday by the Victorian Joint Counter Terrorism Team, including one house in West Footscray believed to belong to Cerantonio's brother.

Police said the raids in Melbourne's north and northwest were "not linked to any increased threat or danger to the community".

Victoria Police and Australian Federal Police said they would not comment as the case was before the courts.

The five men were charged over the weekend with making preparations for incursions into foreign countries to engage in hostile activities, a charge that has a maximum penalty of life in jail.

They had been arrested near Cairns a week ago towing a 7m boat to the tip of Cape York from where they were allegedly planning to sail to Indonesia, en route to Syria to fight.

Federal Attorney-General George Brandis told reporters yesterday that would-be Isis fighters could not be allowed to leave the country because Australia was party to a United Nations Security Council resolution that bound members to stop people crossing national borders to engage in terrorism.

Australia's Government raised the country's terror threat level in 2014 in response to the domestic threat posed by supporters of Isis.

Police have since conducted dozens of raids they say have been aimed at thwarting multiple terror plots around Australia, including an alleged plan to attack government buildings and a naval base in Sydney.

Many of those arrested have been teenagers, including a 16-year-old who was charged with plotting a terrorist attack on an Australian Veteran's Day ceremony in April.