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Re:Ask HN: Finding the Australian Aboriginal flag in all art... by pueblito on Monday August 17, 2020 @11:51AM (#24188373)
From that link, "Thomas was successful in establishing his claim to authorship before the Federal Court in 1997."

IANAL, but I believe you're 23 years too late

Re:Ask HN: Finding the Australian Aboriginal flag in all art... by pbhjpbhj on Monday August 17, 2020 @01:07PM (#24189206)
Authorship isn't sufficient for ownership of copyright.

Trademark is probably the issue that's most pertinent. Even if the flag weren't a work owned by Thomas, for copyright purposes, then it seems it's their trademark (registered or established by use).

IANAL