I seem to recall that when this "Multicom" concept first came in many many moons ago there were complaints at RAPACs across the country from day one.
Complaints from traffic at many aerodromes that they were hearing broadcasts on 126.7 from aircraft nowhere near them, broadcasts from traffic at other aerodromes causing confusion because of the same runway IDs etc. etc.
And that's when discrete CTAFs started to be allocated to specific aerodromes and groups of aerodromes in close proximity to take traffic off 126.7, and that practice of allocating discrete CTAFs continues today. And when the level of radio traffic in a particular area starts to interfere with FIA comms, a Broadcast Area is declared
based on the decision made on airspace policy by Federal Cabinet
Dick - mate .... you know pollies wouldn't know
from clay
