You drongoes
121.5 is the easiest distress frequency for us civilian pilots to listen in case an ELT has been intentionally, or unintentionally, activated. If you hear an ELT, you are required to report it to an ATS unit or the RCC.
Sadly 121.5 is also the easiest frequency to raise another flight which may also be monitoring 121.5. However, once you have raised the other aircraft, you should continue your discussion on 123.45, as this is the designated frequency for 'interpilot air-to-air communications'.
As you probably know, its all available in the Aussie AIP, which is of course available online. (For the 121.5 MHz bit:
http://www.airservicesaustralia.com/...p/gen/3619.pdf and for the 123.45 MHz bit:
http://www.airservicesaustralia.com/.../gen/34124.pdf)
Hooroo