Moderate sized to large, up to 9 mm. Body pale, usually with distinctive wood fibre-filled gut. Head capsule golden yellow with slightly darker postoccipital margin, black mandibles, and dark brown mentum.
Dorsal surface of head comprising frontal apotome, fragmented clypeus (bearing SIII) separated from more anterior sclerite (perhaps labrum, bearing Sl and SII) by golden-brown fragmentary pattern on clypeus; sclerites 1-5 otherwise indistinguishable from clypeus.
Antenna 4-segmented, segments 3 and 4 very short; AR 3.1-3.6, blade simple, annulate, extending well beyond antennal apex; style/peg sensillum 8-10 placed subapically on 2nd segment. Lauterborn organs absent. Ring organ in basal 1/3 of segment 1, with 1 or 2 (2nd minute) subsidiary pits adjacent.
Labrum with SI simple, lanceolate; SII simple, SIll apically bifid; all S setae arranged transversely across labrum, with short SIVa and b lateral to SIll base. Premandible Cardiocladius-like, dark, broad and blunt. with one apical and one inner tooth, without brush. Spinulae and chaetulae simple spines. Labral lamellae of 5-6 fine, thin spines just anterior to SI setae. Epipharynx with medially fissured tormal bar, with ""comb"" of 3 rounded lobes on each side; pecten epipharyngis apparently absent, or indistinguishable from 6 robust simple chaetulae; very small chaetulae basales may be present.
Mandible unusually shaped (though aspect-dependent), somewhat conical/ triangular and heavily sclerotised, with outer and inner margins smooth, with short outer (dorsal) tooth, "true" apical tooth subequal to width of first of 2 inner teeth, innermost tooth with notch. Seta subdentalis very narrow. Seta interna with 4-5 narrowly serrate branches.
Mentum unique, with broad, medially indented area and 3 pairs of lateral teeth; no evidence of ventromental plates, beard absent.
Body not setose. Anterior parapods separate, with crown of elongate, simple, pale spines and claws. Posterior parapods separate, with apical group of simple claws. Procercus twice as high as wide, lightly pigmented, bearing 2 strong median setae and 7-8 anal setae. Anal tubules short, cylindrical.
Austrobrillia is known from all stages of an Australian species, and pupal exuviae of two taxa from South America (Cranston, 2000). The larva of the eastern Australian species is found especially in leaf bundles and less often in stranded wood, although it isn't clear if it is a true miner of immersed wood. Larval guts contain predominantly fibres of wood (McKie and Cranston, 1998). Records are notably from pools and riffles in clean flowing waters.