I'd known that there were Australian soldiers who'd fought beside Americans in Vietnam since I read Peter F. Hamilton's second Reality Dysfunction novel in high school. That one featured a dead Australian Vietnam veteran possessing a living convict in the distant future.
(It's a long story.)
Something notable about this song is that the Australian protagonist has an experience very similar to that associated with American Vietnam veterans (and veterans in general, although Post-Traumatic Stress Disorder became more well-known in the Vietnam generation). He can't sleep at night due to the nightmares, the news helicopter frightens him, and "the rash that comes and goes" might be caused by Agent Orange.
And thanks to "I Was Only Nineteen," I found "Poor Ned," which is about the Australian outlaw Ned Kelly.
The song conflates Kelly with the Eureka Rebellion ("hoist the flag of stars"), in which a bunch of Australian gold miners rebelled against the unpleasant British colonial government. Imagine the Battle of Bunker Hill, only if the Patriots got crushed immediately after inflicting little damage on the enemy but were found innocent of any crime due to jury nullification and ended up getting into elected into the government.
Eureka was many years before Ned Kelly, but Kelly did release a manifesto denouncing the government and calling for justice for the Australian poor. The song lyrics tie in heavily with the manifesto, including defending his shooting of some police (he claims self-defense) and claiming mistreatment of his family by the police. I haven't read Eric Hobsbawm's Bandits, but I do know what a "social bandit" is and Kelly seems to fit the description.