Out of curiosity I’ve spent a few minutes with Alec Wood’s 1997 paper about Australian Amanitas, looking for species with a saccate volva, no annulus and white, serrate gills. There were species in two sections, as follows:
Spores inamyloid Section Vaginatae (spores globose to subglobose: Amanita pallidobrunnea - serrate, Amanita sordidobubalina - gills may be slightly serrate) Section Ovigerae (spores ellipsoid to elongate: Amanita pallidochracea - gills finely serrate).
If you allow ‘annulus often fragile’ you also get:
Spores inamyloid. Section Amidella (pilepellis typically white or brown, spores broadly ellipsoid to elongate: Amanita angustispora - gills very finely serrate).
Don’t know if your sighting belongs to any of these.
I have created the 'serrate gills' pseudo-species to cater for this (and any future) sightings of such strikingly-gilled Amanitas. I wanted to be able to find them easily in the future.