Unexpectedly, the crueler, more savage and difficult world, according to the film, is that of the modern and "civilized" man. In the desert, at least, you can understand your enemies.
Soon, the girl and boy encounter an Aborigine teenager (David Gulpilil) on a "walkabout," a rite of passage in which young men trace the heritage of their ancestors on the land.
As noted above, Walkabout is a comparison of disparate worlds. To achieve that comparison, Nicolas Roeg uses a variety of visual symbols in Walkabout to suggest the corruption -- or at least strangeness -- of the so-called "civilized world."
Early in the film, for example, we see Agutter's character setting-up a blanket and picnic lunch out in harsh desert; clearly a misguided attempt to tame the unspoiled Earth. While she imposes mankind's sense of order on the desert, the film cross-cuts to views of lizards and other inhabitants, going about their business, oblivious to her attempts.
In the same scene, the girl's father goes crazy after Roeg cuts to insert shots of work papers: seemingly endless alphabetical lists of minerals and sheets of byzantine maps. The visual implication set up by the editing is that the father's madness is caused by his job; that the pressure (represented by his work papers) makes him irrevocably snap. The civilized world has made him deranged.
This critique of civilization recurs throughout the film. For instance, as mentioned above, the boy and the girl find an oasis of life in the desert -- water and food -- and without thought of consequences, use it up in a matter of days. When they leave, the land is dry; the fruit is stale and only snakes inhabit the tree.
Perhaps more plainly, the destruction of the desert oasis and its resources is referenced late in the film when the boy and the girl come across a similar setting, writ large: a virtually abandoned mining town.