I ran this past a fellow historian who is an authority on third level airlines. He comments:
To the best of my knowledge Flinders Island Airlines would be the last having switched to Tullamarine from Essendon as from 5 June 1989 and operated from the Australian Airlines Terminal until they closed with last flight 25 May 1990. Southern did operate odd flights into Tullamarine with the Titans but really, the last regular timetabled piston engine aircraft has to be Flinders Island.
Any other comments or suggestions will be welcome.
Rgds