Abdullah Elmir, from the Sydney suburb of Bankstown, is seen in the group's latest video entitled "An Evening on the Banks of the Tigris River in the Province of Nineveh in the Islamic State".
The 17-year-old is seen holding an AK-47 alongside black-clad jihadis praying and reciting religious texts.
In the video, the men say they will "strike the necks of the infidel and Arab countries".
Elmir had gone missing in June, saying he was going fishing. But shortly afterwards told his family he was in Turkey and about to "cross the border".
Last week he appeared in a clip posted on Youtube alongside IS fighters, threatening Prime Minister Tony Abbott and any nation that stands in the group's way.
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