There’s no question he designed it (although that artwork that contains a very similar design is very interesting). But the Government should have probably compulsorarily acquired it by now if he isn’t willing to give it up... I guess they’re waiting for him to die so it won’t be as easy for them to be dragged to the high court over the “on just terms” clause in the Constitution.
My reasoning is that they're a lot more likely to have another "The Castle" episode (great movie, by the way) while he's alive rather than when the copyright is assigned or inherited to somebody else after his death.
I don't see any mentions of it.
https://en.wikipedia.org/wiki/Section_51(xxxi)_of_the_Consti...
Except maybe the US flag in the US (given the particular US prohibition on copyright of federal government works), I don't see why not; and, actually, if the US were to adopt a private design as the official national flag rather than commission it, I'm not sure it would be a problem even in the US, since other private works adopted as national standards but not created by government in the US remain protected by copyright.
There are certainly ways a private copyright on a national flag is inconvenient, but inconvenient and impossible aren't the same thing.
"I, PHILIP MICHAEL JEFFERY, Governor‑General of the Commonwealth of Australia, acting with the advice of the Federal Executive Council and noting the fact that the flag reproduced in Schedule 1 and described in Schedule 2 is recognised as the flag of the Aboriginal peoples of Australia and a flag of significance to the Australian nation generally, appoint that flag, under section 5 of the Flags Act 1953, to be the flag of the Aboriginal peoples of Australia and to be known as the Australian Aboriginal Flag with effect from 1 January 2008."