Dont forget he had been cleared to 3000' and had never been given clearance to descend.
From the preliminary report:
Melbourne Control, Go Cat 6207, going around, climbing runway heading to 2,500.
Go Cat 6207, still identified, climb to 3,000.
3,000, Go Cat 6207.
Go Cat 6207, when able advise intentions.
Go Cat 6207, what would you like to do now?
Approach Go Cat 6207, we had a bit of a downwind there on the landing, are we okay to make a teardrop for landing left turn out but a right turn back to land?
Go Cat 6207, affirm, so come back visual or by the VOR?
No no, we are visual, we are happy just to make a left hand teardrop to turn back into a right hand pattern.
Go Cat 6207, that's approved.
Go Cat 6207, report turning inbound for descent.
Now there is a fair argument that there are grounds for understanding that exchange (the last few sentences) to be understood as having been cleared for a visual approach.
"Go Cat 6207, that's approved." 
, and that he was only requested to report leaving 3000' - which they did.
Given the aircraft was being navigated visually in good visibility to the pilot's home base ...
... well, it's more of a technical breech than a safety one.