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The submarine Monowai volcanic centre lies in the northern Kermadec arc, near the collision of the subduction zone with the Louisville ridge. It consists of a 1500 m high stratovolcano (the Monowai cone) and a 8x11 km caldera (Monowai caldera). Monowai cone is one of the most active in the Kermadec arc as demonstrated by observation of discoloured water, T-wave data and seafloor differences from repeat swath surveys [3, 4]. The eruptive products are tholeitic basalts.[2] Monowai caldera shows signs of ongoing hydrothermal activity [2] but the time of the last eruption is unknown. The eruptive products range from basalt to andesite. Our combined analysis of new swath bathymetry and potential field data shows that the caldera is underlain by a large mafic intrusion (the Monowai pluton) and that a small, recently emplaced magma chamber might exist beneath Monowai cone. The volcanic centre lies inside a graben part of an en-echelon horst graben system spanning the length of the arc. Extension affects the morphology and the dynamics of magma supply.