Radar coverage in Australia is only available close to major cities i.e Melbourne/Brisbane/Perth/Sydney/Adelaide/Darwin/Cairns etc. Beyond these areas a/c must rely on procedural methods for traffic seperation i.e waypoint reporting. Most of WA/NT/QLD/SA is outside radar coverage and outside VHF coverage so they use HF.
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.