My initial reaction too was that it was the Order of Australia …. but then I started looking at the brooch more closely.

All of the Order depictions that I can see have a St Edward’s crown surmounted either on top of the disc or above the central ring (the ring often enameled with the word AUSTRALIA and laurel leaves).

I do not see the crown and, while there is a central element of some sort, there does not seem to be a central ring.

Also, it looks to me a though the disk of the Order has a more defined outer border than this brooch.

Lastly, HM is usually pretty meticulous about how her orders are worn and I find it surprising that she would wear this so casually. It does not fit to me.

I think this is more of a mystery than we initial thought?????