Green’s Dictionary of Slang

bush telegraph n.

also bush wire
[mid-19C] (orig. Aus.)

1. a member of a bushranging gang whose task is to keep his colleagues informed of the whereabouts of potential victims or efforts to capture them.

2. (also blackfellow telegraph, bush telegram) a network of gossip and rumour that brings news, often inaccurate, before the official sources [SAusE blackfellow (now derog.), a Native Australian].