Lobelia koolauensis is a branched shrub reaching 1–2.5 meters in height, with a stem 1–1.5 meters tall and 1.8–3 centimeters in diameter, marked by 3–4 mm tall by 6.5–10 mm wide transversely rhombic cicatrices. It has simple, alternate leaves that are linear, oblanceolate, or narrowly elliptic, 11.8–20 cm long, and 0.9–2.8 cm wide, with an acute to slightly acuminate apex and attenuate base. The leaves are glabrous or have pubescent lower surfaces along the midrib, with distinctly revolute margins. It features an inflorescence that is 0.5–1 meter tall, 2–7-branched, with each branch bearing 12–25 flowers, cylindrical in outline, and green and glabrous; it includes sterile bracts that decrease in size towards the top. The flowers are perfect, fragrant, axillary, and borne on short pedicels. The calyx has five lanceolate sepals, 4.5–6 mm long, while the corolla is zygomorphic (bilaterally symmetric), dorsally cleft, 58-78 mm long, white, and glabrous. It has five stamens with filaments attached to a nectary disk or corolla base, an inferior ovary, and a two-lobed stigma. The fruit is an 18–21 mm long by 9–10 mm diameter loculicidal capsule containing numerous honey-brown seeds, 1.5–2.0 mm long and 0.8–1.2 mm wide, with a short to spatulate dicotyledonous embryo embedded oily endosperm. It is endemic to montane wet shrublands between 670–860 meters.