Kings Crown-indicates pre 1953 manufacture but could be made later if its supposed to represent an earlier period-probably unofficial as no lettering or motto legend on or near the eagle but thats really just a guess.
You asked me in the WWI badge post if I had any idea on this.
I don't know for sure, but if you want a guess... it looks to me as if it's been made up from the metal parts of an officer's cap badge and mounted on a pin. The soldering on the back looks way too crude for a professional job.
As to why, I would go with a ladies hat pin, perhaps for a wife or gf of an RAAF pilot, or something like that. It's too large for a man's tie or lapel pin I think.
Anyway, those are my thoughts...
I have to agree with Rob.
It is not official, and looks like sweetheart piece made of parts
that are crudely lead soldered instead of using silver solder,
which is what a jeweller would do.........
Interesting! Thanks for the input gentlemen.. Found it at the bottom of a drawer in my house! It's been there for years!