The boy, identified in the video as Abu Khaled, and flanked by other young-looking men, addresses the camera, and in several intercut rants threatens to kill "tyrants" from the US, the UK, and "especially Australia".
"To the leaders, to Obama, to Tony Abbott, I say this: These weapons that we have, these soldiers, we will not stop fighting, we will not put down our weapons until we reach your lands, until we take the head of every tyrant and until the black flag is flying high in every single land," he said in the video, which has since been removed from YouTube.
ABC News reported that the spokesman is Abdullah Elmir, who went missing from west Sydney four months ago with a 16-year-old friend. Elmir seems to have quickly climbed the ranks of ISIS, while the other boy was "intercepted" by his father.
Australian prime minister Tony Abbott said the latest video "highlights the threat posed" by ISIS.
Shadow attorney general Mark Dreyfuss told ABC News: "It is tragic that a misguided young Australian has become caught overseas up in this terrorist cause."