I can't recall the last piston RPT into YMML, but do recall a few oddities that others may have forgotten.
Anybody remember OK Airlines (Ovens & King - as in the rivers) that ran a tired looking Aztec from Wangaratta?
Gawne Airlines ran from Shepparton; was it a Baron?
Pacific Aviation ran a Chieftain from Sale to YMML, mid 80's.
Geelong Air Travel operated a Chieftain and Islander, and sometimes a Partenavia.
For info last Kendell Heron ops were early 1981, so I suspect they were not the final piston operators.