Metrosideros tremuloides is a shrub or small tree. The bark is pale, fissured, and flaky, contributing to a distinct rugged appearance. The vegetative buds are narrow and pointed, and the branchlets are slender and spread. The leaves are simple and arranged oppositely. Blades are lanceolate, ranging from 2 to 4.5 cm in length and 0.5 to 1.5 cm in width, featuring an acute apex and a cuneate to attenuate base. The surfaces of the leaves are glabrous, and the margins are entire. Notably, the midrib and the petioles, which are 6-10 mm long and 1-1.5 mm wide, are often red, adding a distinct color contrast to the green of the leaves. The petioles are typically oriented at right angles or more relative to the stems. Stipules are vestigial or absent. The flowers are organized in inflorescences of 3–4 pairs of cymules. These are either glabrous or exhibit short, appressed pubescence. The peduncles are slender and often positioned at right angles to the axis, about 7 mm long and 0.7 mm wide. Pedicels are approximately 4 mm long and 0.5 mm wide. The bracts are ovate, around 4 mm long and 2 mm wide, and are caducous. The flowers are bisexual (perfect) and actinomorphic (radially symmetric). The hypanthium, well-developed above the ovary, measures 2.5–3 mm in length and about 4 mm in width, while the fruiting hypanthium extends to 5–6 mm. Sepals are triangular and approximately 1.5 mm long and 2 mm wide. Petals are obovate and measure about 3 mm in length and width. Stamens are numerous, ranging from 1.5 to 2 cm in height, with distinct filaments and dithecal anthers. The ovary is nearly superior, featuring axile, peltate placentas with numerous anatropous ovules. The style stretches 2–2.5 cm long and terminates with a small stigma. The fruit is a capsule enclosed by the hypanthium and exserted. It is found on steep slopes between 125-700 meters.