Kaihū Forest is a protected forest area located in the Northland region of New Zealand, near the Kaihū River and the settlement of Kaihu.

[1] The forest spans approximately 2,400 ha (5,900 acres) of highlands, including Tutamoe, the second-tallest mountain in Northland, and is home to a wide range of native flora.

[2] Historically, Kaihū Forest was home to kauri trees, which thrived in the region's mix of soft sedimentary material — which allowed for bigger root systems and moisture retention — and igneous material from the Waipoua volcano, which increased the soil's fertility.

[3][4] Early records of the forests in the area include mention of a large amount of kauri, including one by the name of Kairaru that, at a girth of 20 metres (66 ft) and height of 50 metres (160 ft), was the largest kauri ever recorded in New Zealand until destroyed by fire in 1891.

[3][5] Kauri have declined significantly in the area, remaining only in isolated pockets that are threatened by kauri dieback, but have been replaced by a wide range of other native plants, including tōtara, mānuka, kahikatea, and northern rātā, among other species.