Well, our dominant carrier claims 99.3%:
"Australia's largest 3G and 4G enabled mobile network covering 99.3 per cent of the Australian population."
and then:
"Our 4G covers more than 66 per cent of the Australian population today – growing to 85 per cent of the Australian population by December 2013."
Telstra - Coverage & Networks - Mobile
But that doesn't mean your handset can still get a good quality signal. Where one user may get 5 bars of signal and 10mbps downloads, someone else may get only 1 bar and 20kbps downloads. Yet they will all have 'coverage'.
Call me old fashioned, perhaps, but I think mobile phones should work great as phones first. If they can also work as still cameras, video cameras, FM radios, GPS receivers, routers, portable PC's, and NSA citizen monitoring devices, then that's a bonus too, I guess. But it's all secondary if your signal strength is poor.