Acacia aulacocarpa is a shrub to a slender, large tree, 3-40 m tall; trunk up to 1 m in diameter, sometimes fluted. Bark hard, sometimes brownish, about 1 cm thick, longitudinally fissured, peeling in long strips.
Phyllodes straight or falcate, acute or subacute, 5-15 x 0.6-3.5 cm, 3-12 times as long as wide, glabrous, greyish-green or dull grey, with 3 prominent longitudinal veins somewhat crowded towards lower margin at base, usually not yellowish, and with many parallel, not anastomosing, secondary veins, pulvinus, 4-6 mm long, with an ellipsoid basal gland.
Inflorescence a spike, 2-6 cm long, yellow, 1-3 together, peduncle 2-8 mm long, scurfy; flowers pentamerous, bisexual, calyx broadly cupular, 0.5-1 mm long, membranous, with broad, obtuse, scurfy lobes 0.2-0.3 mm long, lobed to the middle, glabrous, 2-3 times as long as the calyx; stamens many, 2.5-3 mm long; ovary 0.5 mm long, shortly pubescent or scurfy.
Pod oblong, up to 10 x 2 cm, light brown, coriaceous to subwoody, with prominent obliquely transverse, dark brown veins, glabrous, often twisted when old. Seed elliptical-oblong, 5-8 x 2.5-3.5 mm, shiny black, transverse or oblique in pod, with pale terminal aril.