Commonsense states that in areas like north of Birdsville the best frequency would be 121.5 as you can get out an emergency call at any time to a high flying airline aircraft. Also ATC can call you if necessary if you have flown into restricted airspace via a high flying airline aircraft as a relay.
Here's what the experts say, at this website:
Aviation search and rescue education - Australian Maritime Safety Authority (AMSA)
The most important things to improve your chances of survival, and to help search and rescuers are:
1. Know how to handle an emergency, forced landing or ditching.
2. Before departing, submit a flight plan, SARTIME or leave a Flight note with someone responsible. If you have to make a forced landing, a major confidence booster to survival is knowing that a search will have commenced.
3. In the event of an emergency, get out a MAYDAY or PAN call. If not operating on an ATS frequency, always have the area or overlying airspace frequency set for immediate use. This is the most responsive method to alert the search and rescue system.
4. During an emergency, after completing the pilot actions and communications calls, activate your 406 MHz distress beacon (ELT, PLB or EPIRB). Make sure it is registered with AMSA.
5. Make sure you have survival equipment suitable for the area being overflown. Know how to use it, and make sure you keep it well maintained. An emergency supply of drinking water is crucial. Also ensure that you have an emergency supply of prescribed medications.
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BTW: If you activate your 406MHz beacon, it transmits on 121.5 ...