I posted this on the Minelab Explorer forum, since that's where most of the Aussies hang out, but got no help there. Thought I might try here. It's 22mm diameter and is one piece. "SOUTH AUSTRALIAN" across the top. The bottom looks like it might have said something, but is warn off? It is very similar to some other states' Police uniform buttons. The back says "Bennett & Son - London". Looks like it might have been silver/chrome plated at one time.
Thanks,
Toby
Thanks,
Toby