I'm American, but it seems to me that when I’ve encountered Australian speech or writing, I didn’t have much trouble understanding it. The words are mostly familiar to me. So what’s going on in Waltzing Matilda?

An excerpt:

Once a jolly swagman camped by a billabong
Under the shade of a coolibah tree,
And he sang as he watched and waited till his billy boiled:
"Who'll come a-waltzing Matilda, with me?"

Chorus snippet

Down came a jumbuck to drink at that billabong.
Up jumped the swagman and grabbed him with glee.
And he sang as he shoved that jumbuck in his tucker bag:
"You'll come a-waltzing Matilda, with me."

I think that a typical American reader or listener will understand almost nothing of the story without discussion.

Was it deliberately written with a lot of Australianisms? Or does the use of a lot of Australianisms reflect something about social class, the way Cockney English would? Do Australians think it's funny because of this quality?