The ATSB has released its report on the slow depressurisation of VH-EBK requiring a diversion to Adelaide on 5 February 2021.
AO-2021-005
The aircraft was south of the Eyre Peninsula heading towards Perth at the time of the incident and in the Melbourne FIR. The report states that just prior to the incident, the aircraft had moved out of range for VHF communications (presumably with Melbourne ATC). PAN PAN and MAYDAY calls were made and acknowledged on the high frequency band with Brisbane ATC.
Does Melbourne ATC have high frequency equipment? There's no further discussion in the safety analysis and findings section of the report.