Unarmed shrub or small tree, to 15 m. Leaves: rhachis bearing glands on upper side, not only at the junction of each pair of pinnae but at other points as well. Leaves with c.12-20 pairs of pinnae; leaflets to 4 mm, numerous. Flowers pale yellow. Pods beaded, with 3-12 segments.
A very fast growing species, flowering and producing large amounts of seeds within a year. The germination of the seeds is stimulated by fires. It is very similar to A. dealbata, another Australian invader, which lacks the glands between the top pairs of pinnae. The leaves of A. dealbata are grey-green, giving the trees a much lighter appearance than A. mearnsii. When in flower, the two species are more easily separated as the flowers of A. dealbata are bright yellow, while those of A. mearnsii are very pale yellow or cream. Also see A. melanoxylon for comparison.