Melicope hiiakae is a shrub or tree that is minutely puberulent when young and becomes glabrate as it matures; its leaves, petioles, and branches are glabrous. The leaves are simple and opposite, with elliptic to ovate or obtuse blades. The margins of these leaves are entire, and they are supported by filiform petioles measuring 9–21 mm in length and about 1 mm in thickness. The base of the petiole is characterized by elliptical plates that are cuneate or subrounded at the apex, obtuse or rounded. The leaf blades are 3.5–6.5 cm long and 2–3.5 cm wide. They are glabrous, with a narrowly brown, dorsally sprung middle rib and lateral nerves that ascend parallel to the leaf surface, featuring a sublinear nucleus at a distance of 1 mm from the coalition. Stipules are absent in this species. The flowers are arranged in axillary, single-flowered cymes, which can extend up to 3.5 cm in length when in fruit, with glabrous axes. The peduncles are 3–8 mm long, and the pedicels are expanded below the calyx. The flowers are perfect, consisting of a calyx with four imbricate sepals and a corolla with four petals. It has eight stamens and a superior ovary that is 4-celled and 4-lobed, with connate carpels but distinct at the apex. Each cell contains two hemitropous ovules, with one ascending. The style ends in 4 short stigmatic branches. The fruit is a capsule approximately 25 mm in diameter, deeply 4-lobed up to two-thirds to three-quarters the length of the fruit. It is sparsely puberulent at the base and becomes glabrescent, with the endocarp sparsely aggregated with hirtellus trichomes at the dorsal sutures. The fruit is green, maturing to a combination of green and red. The seeds are ovoid, about 6–7 mm long, with a black seed coat. It is endemic to the Koolau Range, typically found on the southern ridge in wooded areas between 450 – 750 meters.