Have a look at this link, it has a diagram of radar coverage in Australia.
VHF coverage is normally always available in controlled airspace, with the exception of some controlled airspace above FL200 over central Australia and over water.
VHF is always available for air to air, below VHF coverage HF is used. HF may need to be used very close to major centers as terrain may mask VHF coverage.
The following PDF document (717k) does provide more details on the Australian aispace, radar, radio coverage etc. http://www.airservices.gov.au/pilotcentre/projects/LAMP/ConceptofOps.pdf
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