Plants perennial; usually cespitose, occasionally rhizomatous. Culms 30-150 cm tall, 1-2.5 mm thick, erect, terete to flattened, glabrous; nodes 2-4; lowest internodes not swollen. Sheaths smooth or scabridulous, ribbed; ligules 4.5-8(15) mm, acute to acuminate; blades 10-20(37) cm long, 2-4 mm wide, flat or involute, smooth to scabrous, abaxial surfaces scabridulous, adaxial surfaces deeply ribbed, scabrous. Terminal panicles 15-40 cm, erect, sparse, the lower portion partially enclosed in the uppermost sheaths; branches mostly 7-14 cm, ascending or appressed, often flexuous; pedicels mostly 10-20 mm. Spikelets 8-17 mm, with 2-6 florets. Glumes unequal, green, sometimes purplish in the center, with hyaline margins; lower glumes 3.5-5 mm, 3-5-veined; upper glumes 4-6 mm, 5-7-veined; lemmas 5-8.4 mm,7-veined, papillose to scabrous, apices appearing constricted, 2-4-toothed, awned from the upper 1/2-1/3, awns 14-26 mm, geniculate and twisted; paleas 80-90% the length of the lemmas, papillose; anthers 1.3-2.3 mm. 2n = unknown.

Amphibromus neesii is an Australian species that grows on floodplains and river banks and in marshes and lagoons. It was first noticed growing in North America in 1990 in a vernal pool in Sacramento County, California. Its seeds had been found earlier as a contaminant in Trifolium subterraneum seed being imported from Australia. The discovery of living plants is of particular concern because of their ability to invade and survive in vernal pools.

The common name Swamp Wallabygrass has been used in the Flora region for A. scabrivalvis, but that species is South American. Amphibromus neesii, the type species, has a long history of the same common name, and Wallabygrass is clearly of Australian origin.