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", foreign media reports.
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.
Another three people, aged between 21 and 22, were also charged for the same offence on Thursday afternoon.
All face potential life sentences if convicted.