Thank you for your email about overlay approaches. Advice below from CASA’s airways section:
CASA legisltaive instrument 157/14
CASA 157/14 - Instructions ? GNSS as primary means of navigation for NDB and VOR (overlay) approach (Qantas Airways Limited)
permits an overlay approach because it is approved by the aircraft manufacturer and that capability is included in the Aircraft Flight Manual. If your aircraft has that capability and it is included in the flight manual then he would be able to apply to CASA for an approval.
The explanatory statement in CASA 157/14 has the following:
“The current GNSS rules allow the use of GNSS for primary means navigation under I.F.R. and Visual Flight Rules. In addition, modern RNP-capable aircraft have provisions in their flight manual that allow non-precision approaches using RNP capabilities of the aircraft without the specific approach aid being installed in the aircraft. This instrument contains instructions that allow Qantas Airways Limited (the operator) to reduce dependence on ground-based systems.”