Sevdet Besim, 19, was one of five people arrested last April as part of an alleged plot in Melbourne during Anzac Day commemorations for fallen Australian and New Zealand soldiers.
Besim is accused of discussing his plans online with a British boy, who was 14 at the time.
A court in Melbourne heard that Besim wrote that he would "love to take out some cops".
The alleged plot emerged from a prosecution summary of internet communication on March 20 last year. The court was told that Besim and the British boy discussed the suggestion that a kangaroo could be packed with C4 explosive, painted with the Isis symbol and set on police officers.
Besim is accused of planning to run over and behead an officer and shoot bystanders using the officer's gun.
He has pleaded not guilty to four charges relating to planning an attack. A fifth charge was dropped.
The British teenager was detained for life in October.
