Metrosideros polymorpha var. dieteri is a small tree reaching heights up to 7 meters, with glabrous leaves and woolly (tomentose) inflorescence. The bark of these trees is fissured and flaky, with branchlets being obscurely 4-angled. Vegetative buds are unusually large, obovoid, measuring 2-3 cm in length and 2 cm in width, covered with red, broadly ovate to almost orbicular scales that are sometimes emarginate and fimbriate, spanning 1.5-2.5 cm in length and 1-1.5 cm wide. The leaves of this variety are simple and opposite, with blades that are broadly obovate to oblong, measuring 4-6 cm long and 3-4 cm wide. The apex is broadly rounded, and the base ranges from cuneate to cordate. Both the upper and lower surfaces of the leaves are glabrous. The margins are entire and flat, with the lowermost 2-3 pairs of secondary veins being raised and prominent, radiating from the base of the lamina. The petioles are 6-10 mm long and 2-3 mm wide, and the stipules are vestigial or absent. The flowers are arranged in inflorescences consisting of 3-4 pairs of cymules, notable for their dense, woolly, white to pale gray pubescence covering all external parts. Peduncles are 5-18 mm long and 2-3 mm wide; bracts are ovate to almost orbicular, 6-10 mm long and 4-6 mm wide. The flowers are red, bisexual (perfect), and actinomorphic (radially symmetric), with pedicels 2-3 mm long and 2 mm wide; the hypanthium measures 6-7 mm high and 7-8 mm wide. Sepals are triangular and acute, measuring 3.5-4 mm high and 2.5-3 mm wide. Petals are more or less orbicular, 3.5-5 mm long and 3-4.5 mm wide. Stamens are 20-25 mm long. The ovary is nearly superior, with a 25-30 mm long style. The fruiting hypanthium is pubescent, measuring 5.5-6 mm long and 6.5-7 mm wide, with capsules that are only slightly exserted. It is typically found in shrublands and mesic to wet forests between 700-1300 meters.