Australian registrations were originally G-AU after that was allocated to Australia by the International Commission on Air Navigation (ICAN) as a result oif the 1919 Paris Convention.
See:
Registration Markings of Australian Aircraft
The Australian prefix VH- was allocated to Australia by ICAN in 1928.
ICAN became ICAO in 1944 as a result (or because of??) the Chicago Convention.
Disco..........see the airways museum site re VM and VL prefixes!