I have always assumed that Communism had enough mass support in East and Southeast Asia that the size of the force would not be what prevented an external conquest of Australia
I have a sneaking suspicion that a lack of naval and long range air assets will prevent an invasion.
However, what would less direct pressure (e.g. for favourable trade deals) by a uniformly Communist southeast Asia have done to Australia?
Some kind of CIA coup against a centrist Labor PM? Kind of a soft Chile?
Would it have drawn Britain and the United States in even when the Communist bloc was not attempting an invasion?
Australia's foreign policy, of paying "the insurance premium" in blood, is predicated on this fact. ANZUS, however, does not require any of the contracting parties to do more than consider.
Would internal politics (like Aboriginal affairs) in Australia have become linked to conflicts over relationships with a wholly Communist Southeast Asia (I ask this because of my familiarity with such Trotskyist groups as Socialist Alternative, the International Socialist Organization, and the Democratic Socialist Party)?
These three groups (two now defunct) represent broadly the Australian post-1990s left. Trotskyist. Student focused. Pathetically parliamentary or "social" rather than industrial. They are not a good representative of the state of the Australian far left prior to 1989.
Prior to 1989 Australia's left was composed of the left of the ALP and labourist Unionists, the CPA/Tankies (Czech the reason for the split

and the Maoists. All of whom were MUCH more serious, much more industrial and oriented towards middle class intellectuals and the ALP, rather than students and social causes.
The CPA had to be lectured early in its history about Aboriginal rights by the International, this is due to the White Wages / Stolen Land themes inside Australian labourism. (It is easier to consider the CPA as an alternate labour party, rather than a communist party.) While the CPA did attempt to break this down, the assistance provided to indigenous activists tended to be training rather than movement control. The CPA had some quality of fighting "white australia" until… well until right wing emigres from Eastern Europe were brought in by the ALP. Then it got messy.
While the communist movement (CPA / Tankies / Maoists) from the late 1950s were much better on aboriginal rights, Aboriginality needs to be understood not so much as a "colour" line, but as a "culture" line. Depending on how people of aboriginal descent situate their aboriginality as culture, they can be accepted as "white" or cast out as "black." Similar processes happen with "wogs" and "asians" in the period. But unlike ethnicity, aboriginality has the latent threat of land rights backing its persecution.
The biggest issue for internal politics will, of course, be the chockos, the chocolate soldiers. Nobody will want conscription to fight the red/yellow peril in West Papua. Historically this is what radicalised the Australian working class politically in the late 1960s.
Now the industrial radicalisation will still occur as the "growth" economy is even more under pressure.
So that's why I suggest an Allende/Pinochet solution to Australia's internal politics is likely.
yours,
Sam R.