Plants perennial, stoloniferous.
Culms 17-60 cm tall, 0.5-2 mm thick, geniculate and decumbent at the base; basal branching extravaginal; nodes swollen, dark brown to nearly black; internodes mostly glabrous but retrorsely lanate below the infloresence.
Leaves: sheaths 2.5-5 cm long, glabrous or retrorsely hairy, lower portion becoming fibrous with age; auricles 0.1-.5 mm, glabrous, barely clasping; ligule 0.25-0.6 mm, lacerate; blade 6-25 cm long, 0.5-3.5 mm wide, flat, abaxial surface with midrib and 2 lateral ribs conspicous, white, adaxial surface hirsute or scabridulous on the ribs.
Inflorescence spikelike3-13 cm long, subtended by a small, hair-fringed bract, with 4-13 nodes, each node with a solitary spikelet, spikelets overlapping; rachis internodes 4-5 mm long at the middle of the inflorescence, scabrous on the margins, antrorsely lanate or glabrous on the surfaces.
Spikelets 10-14 mm, solitary, pedicellate, with 4-6 florets, tangential to the rachis and widely diverging at maturity; pedicels about 0.3 mm, hairy.
Glumes unequal, shorter than the spikelet, narrowly to broadly triangular, adaxial surface hairy, abaxial surface scabrous distally, veins 3-5 veins, eccentrically keeled, acute, lower glume 2.5-4 mm long; upper glume 3.7-5.5 mm long. Callus 0.25-4 mm long, with short, stiff hairsLemmas 9-11 mm long, 5-veined, obscurely keeled on the lower portion, scabrous, margins narrowly herbaceous, awn-tipped, awn 1.5-4 mm, canaliculate at the base. Palea 5.3-7 mm long, keels denticulate, stiffly hairy between the keys. Anthers 2.4-2.75 mm long. Caryopses 3.5-4 mm long.
There are two subspecies.
Australopyrum calcis subsp. calcis is stouter, has leaf blades that are hairy on the adaxial surface, antrorsely hairy rachises, and lemmas that are more scabrous than A. calcis subsp. optatum
Australopyrum calcis is known from only three areas. It apears to be confined to limestone habitats.
subsp. calcis: South Island, Marlborough
subsp. optatum: South Island, North Canterbury