I think there's some big misconception about class E here. It's en-route controlled airspace for IFR aircraft that's it, it's not designed for terminal airspace around an Aerodrome, and therefore is not suitable for this purpose.
Class E airspace is meant to capture IFR as soon as possible after airborne from non towered aerodromes that don't qualify for a tower. It would be ridiculous to have a tower at Kingaroy but it makes 100% sense to get IFR aircraft into CTA ASAP from Kingaroy. IFR airworking at Kingaroy do not need to be IFR in VMC.
One of the major reasons it's not suitable for terminal areas is that VFR can operate how the hell they want and the closer you get to an airfield the more erratic and dense traffic gets, which means clearing IFR on strict routes no longer works when they have to constantly change course or altitude profiles to fit in with other traffic.
So hands up anybody that wants Class E associated with a high density control zone?? Anyone?? Beuller?? Beuller?? E over D will work in some circumstances, C over D will be more appropriate in others.
This is also the reason Class E gets dicey at lower altitudes depending on the density of traffic in an area. Imagine setting departure paths for three or four IFR aircraft and then find a tribe of VFR trainers blocking the path, because they departed at the same time or just ahead of the IFRs and are now in the way.
ATC will separate the IFR from IFR. 3 or 4 VFR aircraft blocking an IFR departure path. GA, in Australia??? 3 or 4 VFR aircraft in Australia, that's a laugh, $3.20 a litre, thats $12.80 a gallon. GA is becoming a distant memory, you won't have to worry about them for too much longer.
The radio traffic and workload is now through the roof. (at Ballina maybe) And IFR pickup will not work for RPT above 5700kg, as they are not allowed to operate VFR, so you'd have to write a new rule for them to depart with some sort of schedule.
If you can't depart VFR, don't.
At present if I arrive at an airport with high VFR in class G I'm passed traffic and manuvre as required to fit in with it, very simple communication over the CTAF, make it class E and I need approval to deviate from cleared routes, doubling the radio traffic and requiring talking on two frequencies and if there's any IFR traffic blocking ATC margins I won't get clearance.
Where is this aviation nirvanna in Australia you speak of? Where there are plentiful VFR's, so many that an IFR aircraft find it impossible to manouvre around? Are you sure you're not dreaming, in another country, where aviation lives and thrives?
I also agree with the statement above regarding those that want to remain OCTA for direct tracking, less delays or whatever.
Yep, either do E properly or leave it G.