Metrosideros polymorpha var. pumila is a prostrate shrub typically less than 1 meter high. It can either be simple or have one or two ascending branches. The stems are either unbranched or sparingly branched with a gray outer bark that peels off in shreds. The leaves are simple and opposite, with broadly ovate or sometimes obovate blades, the largest being about 5 cm in diameter. The apex ranges from rounded to sometimes obtuse or acute, while the base is cuneate to cordate. The upper surfaces are glabrous, light green, and shiny, whereas the lower ones are also dull and glandular. Margins are lightly induplicate (revolute), featuring several pairs of raised veins radiating from the base, with the midrib being impressed above and prominent beneath. Petioles are stout and about 6 mm long. Stipules are vestigial or absent. The flowers are arranged in inflorescences of 2–5 pairs of cymules. The cymes are large, with peduncles densely woolly and measuring 2.5 cm in length. Bracts are broadly ovate to suborbicular, 5–10 mm long and 3–5 mm wide. The flowers are red, bisexual (perfect), and actinomorphic (radially symmetric); pedicels measure 2–8 mm long and 1–2 mm wide; the hypanthium is 3–7 mm high and 3–8 mm wide. Sepals are very woolly except for the short, triangular lobes, which are almost glabrous but glandular. Petals are red, broadly obovate, about 6 mm in length, glandular on the outside, and ciliate. Stamens are numerous, 2.5 cm long, and dark red. The ovary is nearly superior, deeply immersed at the bottom of the calyx, with a disk-like top that is very glandular and resinous. The style measures up to 2.5 cm long, not enlarged at the stigmatose apex. The fruits are capsules that are slightly included to exerted; the fruiting hypanthium is 3–8 mm long and wide, either pubescent or glabrous. It typically inhabits mid to high-elevation swamps.