Metrosideros rugosa is a shrub or rarely a small tree. The stems are branched from the base, with pale bark that separates in thin flakes. The branchlets are quadrangular and narrowly winged. The vegetative buds are approximately obovoid and adorned with many fimbriate rounded scales up to 10 mm long and 7 mm wide. The leaves are simple and dorsiventral, showing a clear difference between the upper and lower surfaces. They are arranged oppositely with obovate to suborbicular blades measuring 2–2.8 cm long and 1.3–2.2 cm wide. The apex is rounded, and the base ranges from cuneate to rounded. The upper surfaces are dark green, shiny, and glabrous, while the lower surfaces are densely woolly with tan to reddish-brown, long, appressed hairs. The margins are revolute. The midrib and secondary veins are strongly raised on the lower surface and impressed on the upper surface. Petioles are approximately 2 mm long and 1.5–2 mm wide. Stipules are vestigial or absent. The flowers are arranged in inflorescences of 2–3 pairs of cymules covered with a dense, woolly, tan tomentum. Peduncles are less than 5 mm long and 2 mm wide, and pedicels are less than 2 mm long and 1.5 mm wide. Bracts are elliptic, less than 3 mm long and 2.5 mm wide. The flowers are bisexual (perfect) and actinomorphic (radially symmetric). The hypanthium is well-developed above the ovary, with the fruiting hypanthium measuring approximately 5 mm in length and width. Sepals are distinct, rounded to triangular, and about 2 mm in size. Petals are distinct, obovate, less than 3 mm long, and 2.5 mm wide, with densely fimbriate margins. Stamens are numerous, measuring 1.8–2.3 cm long, with distinct filaments and dithecal anthers. The ovary is nearly superior, with numerous anatropous ovules and axile placentation. The style is 2.3–2.7 cm long, terminal, and simple, with a small stigma. The fruits are capsules. It is found on or near the summits of cliffs between 700-800 meters.