BRIEF DESCRIPTION: The following description is based on that of Bas (1969).

The cap of Amanita grossa is about 100 - 140 mm wide, convex to plano-convex, soon with flattened or somewhat depressed center, fleshy, white, with nonsulcate, appendiculate margin.  The cap is scattered with thin, white, irregularly shaped, crust- to patch-like, rarely wart-like, vaguely delimited, subfelted-subpulverulent remnants of volva.

The gills are adnexed to adnate, very broad, and probably white.

The stem is about 75 - 100 x 20 - 30 mm, equal or slightly tapering upward, solid, white, exannulate, with some thin, subfelted patches of volva on a transitional zone between the stem and the bulb.

The spores measure 11 - 12.5 (-13.5) x (6.5-) 7 - 8.5 µm and are amyloid and ellipsoid to elongate.  Clamps are present at bases of basidia.

Amanita grossa was originally described from Tasmania. Bas (1969) knew it only from the type, but considered that it might occur on the mainland of Australia.