Naaaaah, Ozzies don't mumble…they moan and whinge…and tell on each other
Ozzie RT standards are generally very good IMHO, yes there are some dumbass local interpretations…"Daaaayseemaal" (but they exist all over).
The only serious thing I've ever noticed personally is the speed of the read backs at time. I reckon if anyone thinks people mumble on the radio, its most likely due to poor technique with the boom mic.