A program of explosion seismology in central North Island, New Zealand, discovered a strong reflector within the upper mantle. Reflections from this (PmP2) are spatially confined to come from an interface 35 km deep and directly beneath a 40 km-wide, back-arc extension zone with active volcanism, high heat flow, low Pn wave-speeds and thinned crust. On the basis of relative reflection amplitudes, the mantle reflections are most readily explained by an interface with a negative seismic impedance contrast. A satisfactory fit is obtained for a layer with a 40--90% drop in S-wave speed (Vs) compared to the surrounding mantle. We interpret this layer to be a 40 km-wide reservoir of partial melt pooled at a thermal boundary layer within the upper mantle.