ATC, its workload, clarity and wording of the call are all absent from the report. Thailand uses 5 letter registrations (like Australia). Were there any aircraft in the Melb TCA with similar sounding 3 letter call signs? Did the mic clip one of the call sign letters which led to confusion on the part of the captain?
These are valid issues that may or may not have contributed that the ATSB should have considered, but are absent from the report.
With the lack of clarity of the ATSB report, I'm prepared to give the pilot some benefit of the doubt and argue that his response time may have been 15 seconds, not the 50 seconds that is emotively used in the ATSB report.
If there was any hint of confusion about the calls, then both sides of the coin need to be examined, not just the PIC.
I think that it is indefensible that the ATSB do not append transcripts to their reports as is common in other countries.
I think this is a case study of another sub standard ATSB report.