A theme of The Secret River is that the past has an influence on the present. Theme is the story's message.
This theme has two aspects. First, Thornhill ends up sentenced to deportment in Australia. He stole some wood and that was it. He could not return to London.
Once in Australia, Thornhill has to try to find a way to exist. That means farming. He admires the aboriginals, who are also farmer.
But they did not bother to build a fence to keep animals from getting out. Instead they created a tasty patch to lure them in. Either way, it meant fresh meat for dinner. (p. 229)
The interaction between the natives and the newcomers is connected to greater concerns in modern day Australia. In fact, Grenville wrote The Secret River as a result of her research into her family history and recent court battles between aboriginals and whites in Australia today.