Metrosideros carminea prefers warm moist habitats and grows up to 15 m. long or more, with the main stem several centimetres in diameter. The small, glossy, pointed leaves are thick, and often widest in the middle. The small rounded and shiny deep-green leaves have are borne on reddish new stems. Carmine rātā flowers from late winter to mid-spring, and has vibrant displays of bright red flowers in groups at the ends of the stems. The seed capsules ripen between late spring and early autumn. It climbs in the same way as ivy, sending out short adventitious roots to adhere to the trunks of host trees, penetrating and clinging to rough surfaces. The climbing shoots of juvenile plants grow rapidly and quickly extend the length of the plant. The short clinging roots usually die after about a year, so that when the vine is mature, the thick, twisted, rope-like stems hang free from the host like thick ropes.