Have a look at AIC H05/18 and H28/16.
If you are flying a BARO AIDED GPS approach you are flying an RNAV APV approach to LNAV/VNAV minima. The approach is designed, and consequently the acft 'system' is coded with a vertical glidepath that clears obstacles by the required amount from the the Threshold at the Threshold Crossing Height (TCH) back up to the FAF. It is not designed with intermediate not below heights from the FAF vs distance to run as per a normal NPA. As there are no intermediate not below heights, the pilot cannot be responsible for ensuring not below heights are met, the DA is limiting.
If you are flying an approach in acft not certified with BARO VNAV, you are flying an RNAV NPA approach. You can only fly this approach to LNAV minima, or the old (being superceded) S-I GNSS minima label. This is an MDA. The approach is designed with intermediate not below altitudes vs distance to run. There is no vertical glidepath design and therefore no coding of a vertical glidepath to the threshold in the acft systems. As there is no vertical glidepath in the design of the approach - the pilot is responsible for ensuring the intermediate altitudes and finally the MDA are met.
This is where advisory VNAV can be confusing - some aircraft systems provide a vertical representation of the vertical profile on an LNAV only approach, i.e on an RNAV NPA. This is purely an aircraft generated capability, the vertical glidepath is not designed into the approach, therefore the vertical glidepath presented in the aircraft cannot be relied upon for terrain clearance. The pilot remains responsible for ensuring 'not below until x.x miles' is met. Think of it as an automated presentation of the advisory DME/ALT scale.
If you are certified for Baro VNAV - but do not have an accurate QNH or are outside the temp limits of the APV design, then you can only fly the LNAV minima.
RNAV LPV approaches (LPV = Localiser performance with vertical guidance) are the ones that require augmentation - this is not available in Australia. The fundamental design difference that usually enables lower minima on these approaches is that the design tolerances are angular (vertically and horizontally) as per an ILS, vice a fixed distance as per RNAV NPA and RNAV APV.