"Re ICAO - we are a sovereign country and only have to notify a difference."
That after much searching I can not find. But as above it is CASA to implement Annex 6. That includes this "ICAO requires that all aircraft monitor 121.5 at all times in areas where ELTs must be carried (which includes the whole of Australia)."
"CASA is responsible for implementing Australia's obligations under Annexes 1, 2, 6, 7, 8, 10, 11, 14 and 18 of the Chicago Convention"
source -https://infrastructure.gov.au/aviation/safety/ssp/chapter_1.2.aspx