Metrosideros macropus is a small tree that typically reaches heights of 5–7 meters. Its bark is characteristically fissured and flaky, and the vegetative buds are narrow and pointed, featuring long–persistent, obovate to spatulate, attenuate scales up to 22 mm long and 7 mm wide. The leaves are simple and dorsiventral, exhibiting a distinct difference between the upper and lower surfaces. They are arranged oppositely with broadly ovate blades measuring 4.5–7 cm in length and 3–5 cm in width. The apex ranges from rounded to acute or moderately attenuate, while the base is rounded to truncate. The surfaces of the leaves are glabrous, and the margins are entire. Petioles are 15–35 mm long and 1.5–2.5 mm wide, often oriented to the stems at right angles. Stipules are either vestigial or absent. The flowers are borne in inflorescences consisting of 4–6 pairs of cymules, usually glabrous or sometimes bear an appressed silvery pubescence. Peduncles are approximately 12 mm long and 2 mm wide, and pedicels are about 3 mm long and 1 mm wide. The bracts are ovate to lanceolate, roughly 5.5 mm long and 3.5 mm wide, and remain long-persistent. The flowers themselves are bisexual (perfect) and actinomorphic. The hypanthium, well-developed above the ovary, measures 4–6 mm long and 7–9 mm wide in fruit, exhibiting prominent veins and a surface that ranges from glabrous to puberulent. Sepals are triangular and distinct, measuring 2–3 mm long and 2–3.5 mm wide. Petals are distinct, obovate to orbicular, approximately 5 mm long and 3 mm wide, with the hypanthium around 3–4 mm high and 4–5 mm wide. Stamens are 1.8–2.5 cm long, with distinct filaments and dithecal anthers. The ovary is nearly superior with axile, peltate placentas, numerous anatropous ovules, and a 2–2.7 cm long terminal style. The fruits are capsules enclosed by the hypanthium and exserted, measuring 2–3 mm. It is native to the Koʻolau Mountains between 250–730 meters.