Police in Sydney have arrested a 15-year-old and a 20-year-old in a counter-terror operation.

They have been charged with "conspiracy to conduct an act in preparation for a terrorist act", Federal Police said.

The arrests are linked to a plot outlined in material seized last year as part of Operation Appleby.

That operation, in September 2014, was sparked by intelligence reports that Islamist extremists were planning random killings in Australia.

Then-Prime Minister Tony Abbott said at the time a senior Australian Islamic State militant had called for "demonstration killings", reportedly including a public beheading.

Phil Mercer, BBC News, Sydney: "Continue to thwart attacks"

While its FA-18 fighters bomb so-called Islamic State targets many thousands of kilometres away, and its soldiers train Iraqi forces, Australia continues to confront the enemy within.

More counter-terrorism raids in Sydney have yielded further arrests, and will make an uneasy nation even more anxious on the eve of the first anniversary of the deadly Sydney siege.

Last month, a poll found that more than half of Australians thought a large-scale attack was likely, and one-quarter was convinced it was inevitable.

The prime minister has promised a "calm, clinical and effective" response to the menace of home-grown extremism.

"We cannot eliminate entirely the risk of terrorism any more than we can eliminate the risk of any serious crime," said Malcolm Turnbull in his first national security address. "But we can mitigate it. We will continue to thwart and frustrate many attacks before they occur."

Timeline of recent Australia terror incidents

• 11 Aug 2014: Image emerges apparently showing young son of an Australian militant fighting in Syria holding the severed head of a soldier.
• 27 Aug 2014: New counter-terrorism units set up in airports to stop people travelling to join militant groups.
• 14 Sep 2014: Prime Minister Tony Abbott commits 600 troops to international fight against the so-called Islamic State.
• 18 Sep 2014: Australia carries out what it calls its biggest counter-terrorism raids; Mr Abbott later confirms this was linked to an alleged plot by Australia-based individuals to kill a randomly selected member of the public.
• 15-16 December, 2014: Gunman Man Haron Monis, a self-styled cleric originally from Iran, takes 17 people hostage in a cafe in the centre of Sydney. Monis and two hostages are killed after a 16-hour siege. His former lawyer describes him as "unhinged" but not a jihadist.
• 2 April: British police arrest a 14-year-old boy in connection with an alleged plot to target an Anzac memorial event in Australia.
• 7 October: Four people detained in Sydney in connection with the killing of police worker Curtis Cheng.

At least 800 heavily armed officers arrested 16 people as part of Operation Appleby in September 2014, in what was Australia's biggest ever anti-terror operation.

Police said the arrests on Thursday were not linked to a new plot, but to documents seized during those raids that talked about a plan to target government and police buildings.

Three other people already in jail for other offences will also face new charges, they said.

"As a result of putting all of that information together, working through those documents, putting physical and electronic surveillance together, we were able to build a case of conspiracy for five people involved in the preparation of these documents," said Deputy Commissioner of National Security Michael Phelan.

Caption: Deputy Commissioner Michael Phelan said both of the suspects arrested would face court on Thursday

Those arrested on Thursday are also known to those involved in the terror-linked shooting of Sydney police worker Curtis Cheng, police said.

Mr Cheng, 58, was shot dead outside his police headquarters office as he left work last October.

Police shot dead his attacker, 15-year-old Farhad Jabar, at the scene.

New South Wales Police said the 15-year-old boy arrested on Thursday had clearly been radicalised.

Caption: Farhad Jabar, 15, was killed in a shootout with police on Friday after he gunned down police accountant Curtis Cheng

"It is disturbing that we continue to deal with teenaged children in this environment," said Deputy Commissioner Catherine Burn.

"To be putting a 15-year-old before the courts on very serious charges that carry a maximum penalty of life imprisonment demonstrates the difficulties law enforcement face."

Police said a total of 11 people had now been charged under Operation Appleby.