Shrub to 3 m high and 6 m wide, spreading, sometimes almost prostrate or arborescent to 6 m high, resinous. Bark fissured or rarely smooth, grey or brownish grey. Branchlets ± terete, light fawn to dark brown, velvety, tomentose or hirsute. Phyllodes asymmetrical, obliquely ovate-rhomboid or elliptic to suborbicular, (2–) 4–8 cm long, (12–) 20–50 (–60) mm wide, with a strongly curved adaxial margin and ± straight to slightly curved abaxial margin, sometimes with a setose point at apex, coriaceous, ± tomentose mainly along veins and margins, with 3 or rarely 2 or 4 prominent curved veins joining the upper margin at different slightly indented points with lowest vein concurrent with lower margin for several mm and terminating at or just below apiculate phyllode apex; minor veins strongly reticulate; gland 1, basal, prominent. Spikes 1 or 2 per axil, 1–3 (–4) cm long (sometimes reported to 7 cm long, e.g. C.R. Dunlop et al., Flora of the Darwin Region 2: 13, 1995), yellow to golden; peduncles usually to 5 mm long (sometimes longer, to 13 mm long reported by Dunlop et al. 1995), hairy. Flowers 5-or 6-merous; calyx 0.7–2 mm long, dissected to ⅓–½, hirsute, sometimes glabrous near base; ovary densely pubescent. Pods linear, not to slightly constricted between and raised over seeds, ± straight to slightly curved, (1.5–) 3.5–7.5 cm long, (2.5–) 3–5 mm wide, crustaceous, velvety-hairy, breaking into 1-seeded portions. Seeds longitudinal, broadly oblong-to narrowly oblong-elliptic, 4.3–6.5 cm long, blackish brown; areole open, elongate, depressed.
Grows in sandy, shallow rocky, skeletal and lateritic soils, in heath, shrubland, low Acacia woodland and eucalypt woodland, on seashores, hillsides, plateaus, rocky and scree slopes and above gorges, on sandstone, granite or quartzite.
Acacia humifusa features on the $50 Australian banknote released into general circulation from 18 October 2018, as part of the Next Generation of Banknotes featuring Australian native wattles and birds.
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