Abstract: Rainfall and runoff data do not exist in Nakanai Mountains, the main range of New Britain Island, Papua New Guinea. We collected field data during a 4 weeks expedition in the rainforest. In the mountain, the annual rainfall may be about 12 m; the soil is able to absorb a 50 mm rainfall, but a 20 mm shower starts runoff if the soil is saturated by a previous rain and gullies sinking in caves are flooded. Flooding propagates at about 1 km/h inside Muruk Cave. The discharge of the main rivers, Matali and Galowe, correspond respectively in low water to 27 and 33 m3 /s. The ratios between low, medium, flood and exceptional flows are respectively 1, 5, 10 and 50.