A British 15-year-old and an Australian terror suspect planned to fill a kangaroo with explosives, paint an Isis flag on its flank and then set it loose on police officers, court documents have alleged.

The unnamed Briton and 19-year-old Sevdet Besim are alleged to have discussed the bizarre tactic on the Telegram messenger service, as part of broader plans to commit a major terror attack in Melbourne during commemorative services for Anzac Day.

Mr Besim stands trial accused of planning to use a car to run over and then behead a police officer. In court documents presented on Thursday by Melbourne Magistrates Court, he was alleged to have written: “I'd love to take out some cops. I was gonna meet with them then take some heads.”

He and four alleged co-conspirators were arrested in Melbourne one week before last year’s 25 April Anzac Day commemorations, which were to mark a century since the 1915 Gallipoli landings and are more generally held honour Australian and New Zealander servicemen and women.

In the summary of Telegram conversations between Mr Besim and the British boy, prosecutors alleged that “communications resume in the early hours of Thursday 20 March 2015 with an image being sent by BESIM with a comment of 'look what I got ahaha'.

“The conversation continues with BESIM detailing what he did that day and they have a general discussion around animals and wildlife in Australia including a suggestion that a kangaroo could be packed with C4 explosive, painted with the Isis symbol and set loose on police officers.”

Sevdet Besim in a picture posted to Instagram in March 2015 (Sevdet Besim/Instagram)

Mr Besim allegedly also said he was “ready to fight these dogs on there (sic) doorstep” and “Main thing I guess is that I send the dog to hell”.

Police alleged the teenager was motivated by extremism and had expressed support for terrorist organisations, particularly the Isis movement. The court documents claimed evidence he conducted internet searches in February 2015 including “Anzac day parade 2015”, “shrine of Remembrance” and “Melbourne Anzac Day 2015”.

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Mr Besim has pleaded not guilty to four charges relating to planning a terrorist attack, and faces a potential life sentence if convicted. Prosecutors have withdrawn another charge of conspiring to commit an attack.

In Britain the 15-year-old from Blackburn, who cannot be named due to his age, was sentenced at Manchester Crown Court for his part in the Anzac Day plot.