The cap of Amanita mumura is 45 - 64 mm wide, convex to plano-convex, dry, with a white to buff center and an appendiculate margin.  The volval remnants form complete, pulverulent, buff covering, occasionally completely lost in older specimens.  The flesh is pale buff with some yellowing on exposure.

Gills are crowded, free, white, 8 mm wide; the short gills are attenuate.

Its stem is 50 - 100 × 10 - 32 mm, clavate to sub-bulbous, hollow, smooth, buff with yellowish and sienna fulvous stains.  The volval remnants coat the lower stem and upper bulb in pulverulent to sub-felted layer with buff with yellowish and sienna fulvous stains.  In a few specimens a complete "limbus internus" of the volva is left circling the base of the stem.  The ring is friable, felted, disappearing in older specimens, buff to ochraceous.  The flesh is pale buff with some yellowing on exposure.

The entire fruiting body becomes rosy buff on drying.

The spores measure (8-) 9 - 12 × (6-) 6.5 - 8.5 µm and are broadly ellipsoid to ellipsoid, occasionally elongate and amyloid.  Clamps are present at bases of the basidia.

Originally described from the North Island, New Zealand, associated with Southern Beech (Nothofagus), and Leptospermum.  This species is apparently unique to New Zealand.

Ridley felt that this species best belonged in Bas' (1969) stirps Grossa.  Stirps Grossa previously included only taxa from Australia.  It would appear that ancestral members of stirps Grossa may have pre-existed the break-up of Gondwana.