The subduction interface beneath the northern part of the Hikurangi subduction margin on the east coast of New Zealand's North Island is exceptionally shallow and cool compared with other margins where slow slip has been observed. Here we use magnetotelluric data to show that a marked decrease in the conductivity of the fore-arc sediments coincides with the onset of seismicity at ∼10 km depth. Below the sediments, a dipping band of seismicity and intermediate conductivity at the subduction interface connects to a deeper more conductive zone above the down-going plate. This deeper conductive zone is interpreted to be under-plated sediments. These results and results from previous seismic tomography in the area suggest that the intermediate resistivity zone represents a region of upward fluid transport near the plate interface followed by fluid escape into the upper-plate.