because we have the mandatory requirement for transponders in Class E
Not quite - my underlining:
AIP GEN 1.5-9:
6.1.2 All aircraft, except aircraft operating to the VFR which are not fitted with an engine driven electrical system capable of continuously
powering a transponder, must be fitted with a serviceable Mode A/C or Mode S SSR transponder when operating in Class E airspace.
http://www.casa.gov.au/rules/orders/095.htm
Look at parts 95.8 / 95.10 / 95.12 / 95.14 / 95.32 / 95.54 / 95.55. You will see these types are permitted to operate in class E in VMC, and clearly most if not all do not have electrical systems capable of powering a transponder. So the AIP reference backed by CAR (174A?) means to me they don't need to operate a transponder in class E.
Which brings us to the safety of class E airspace debate .......