Sin, evil, and crime
Not to say that sin and crime are the same, but they are thematically related, especially in this novel. The question about "blurred lines" exists throughout the novel. Even the most noble, appropriate person is a petty thief. No one in Australia is innocent (almost by design). So Australia represents the underground of Britain, and they acknowledge that within the book, even performing a play about that very thing.
Community and society
The question that always comes to mind when thinking about Australia's early colonization is, "What is a society of miscreants like?" The book seems to play with that idea, but actually, the truth of the matter is that although Australia is filled with criminals, so is Great Britain, still. If humans are inherently dangerous and evil, then Australia might not actually be any worse, even if they only send criminals. This thematic consideration runs through the entire length of the novel.
Colonialism and British government
One of the first things that the people of Australia start to wonder is, "Why are the British still in charge if they're so far away?" That doesn't mean they aren't British, or that they don't feel British, but rather, it means that Australia is essentially different than Britain and always has been. The question then becomes about Britain's authority. Why does this nation constantly feel like conquering the planet? This story happens far enough away from England that the characters naturally contemplate such things.
