that would be supplemented by monitoring HF
I'm not sure that the C150 had HF
Most of my experience with RPT at non controlled airports is at Mildura and to a lesser extent King Island, where I find the Virgin / QantasLink / Rex drivers very impressive.
I would have expected that the Metroliner would have made enough calls that the C150 would hear at least one which would initiate a dialogue between the two aircraft. There might be a large dollup of coincidence / bad luck here.
But, it would be very understandable / reasonable that the first time the C150 tuned to area was at the CTAF boundary on climb, which really limits the opportunity to hear incoming calls.
Another aircraft may have stayed lower with RPT in the area, but once again, its very understandable that the C150 was trying to get as much altitude as quickly as possible before it was too far from land.
One of the issues that I have found is that VFR & IFR pilots speak different languages and have different maps. VFR pilots tend to reference their position compared with ground features. IFR pilots tend to use distance & bearing from airports, aids or intersections. Intersection names used to frequently give clues to where they are, but post the ground based aid shutdown they are meaningless. A VFR pilot might understand 10nm from Cowes, but is unlikely to have any idea what 10nm from Sunti means. Conversely, an IFR flight may not even have a WAC chart on board and may not really understand something like abeam Egg Lagoon (a real place on track on King Is). Depending on the handheld GPS the C150 pilot presumably had, he would have a distance to run to Barwon Heads, but it may not be quick & easy to get distance from King Island airport, which is what the airline needs. AND the VFR GPS will be measuring distance from the Aerodrome reference (or not even that if the pilot just hit "Direct to"), but the airline will be measuring from the NDB beacon.
I worry that CASA is creating a 4 tiered system with different rules, minimum equipment, common practice and language for each that contributes to incidents like this. Viz: IFR / VFR / RAA / Gliders. I'll bet we never see this as a factor in any ATSB report!!
BTW, am I the only one who thinks that a guy who regularly flies a C150 from Barwon Heads to King Island is a bit of a legend? He's a bolder man than me!