CPDLC in Australian Airspace when under VHF is considered a secondary comms method, and ADS is of course useful.
However, VHF remains the primary communication method when within range. All requests are to be by VHF, unless initiated by ATC on CPDLC.
It is used extensively in the Australian FIR's outside VHF range... eg by MEL when going south, and BNE when north of Australia up towards Ujung, or heading off east, when it becomes the primary, with HF being the backup.
You mustn't be venturing far enough from Australia
(The more western routes from SYD to SIN you are out of VHF for 30mins or so.)