Jacks Statement responding to Leadsled
...CPDLC is only to be used outside of VHF coverage so you should be able to keep listening.
is entirely correct.
AIP GEN3.4 6.3.2
In Australian continental airspace, CPDLC is normally used as a backup communications medium to VHF voice. Depending on traffic loadings, controllers may initiate the use of CPDLC in some domestic airspace sectors. Other than the transmission of position reports as described in paragraph
6.5.1, pilots should not initiate CPDLC messaging within domestic VHF voice airspace unless authorised to do so by the controller, or an emergency situation exists.
What nitpicker330 says is partially correct in that the use of ADS (Automatic Dependent Surveillance) means we don't normally have to give position reports, BUT we don't normally send CPDLC position reports other than to establish the link, in accordance with the AIP quote above.