Ben87r:
Refer to CAAP 166(link below)
Page 21 states the content of a broadcast, which includes intentions. A broadcast immediately before entering a runway (line-up call) would include intentions- ie for circuits, departure to the west etc, upwind departure for Dubbo etc. hence no need for a departure call.
Page 22 contains a table of recommended broadcasts.
https://www.casa.gov.au/sites/g/files/net351/f/_assets/main/download/caaps/ops/166-1.pdf
Uneccessary broadcasts can result in a safety issue when a non-towered airport gets busy. Put 4 or 5 aircraft into a circuit all making taxying, lineup, downwind, base, final, clear of runway calls there's no room for inbound traffic calls. It's also difficult to maintain situational awareness with constant radio chatter - there's a tendency to "tune out", a bit like a nagging partner or whinging child.
It also promotes a mindset of reliance on radio for traffic awareness, remember radios are not compulsory at all airports, radios fail and errors result in incorrect frequencies/selections. Lookout is you primary means of traffic separation, radio only assists.