Escaped convicts who have taken refuge in the Australian “bush,” and subsist by plunder.
“The bushrangers at first were absentees [i.e. escaped convicts] who were soon allured or driven to theft and violence. So early as 1808 they had, by systematic robbery, excited feelings of alarm.” —West: Tasmania.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894