This site provides an excellent tool for predicting possible coverage areas for High Frequencies (HF) between 3 and 30MHz.
Police and Emergency Services in Australia still use High Frequency (HF) radio to maintain communications, particularly in designated remote and outback areas of Australia.
A number of HF travellers use various radio networks with monitoring bases scattered around Australia to provide a safety net when in remote areas. The Royal Flying Doctor Service (RFDS) bases provide an additional safety net with communications and support in medical emergency situations.
Aircraft still carry HF radio for outback and remote communications, although the prominence of satellite linked ground relay stations is increasing.
Although this tool is focused on amateur radio bands (wavelength/frequency) by choosing the closest band to your operating frequency, this should give you approximate coverage predictions at the time you input basic location and power information.
The colour coded map above gives a percentage indicator of the likelihood of coverage from your location around Australia.
This map shows that it is likely that I would be able to communicate reliably across most of the southern and eastern parts of Australia, with a 60-70% chance in far north Queensland and most of Western Australia.
No tool is very perfect but this could prove useful.