A new 3-D seismic P wave velocity model of the Sunda-Banda Arc Transition from local earthquake tomography reveals (i) northward subduction of oceanic lithosphere, associated with the convergence of Australia and Sundaland, as a high-velocity zone extending down to ~200 km depth; (ii) two distinct low-velocity zones, one immediately above the slab, which is likely a zone of partial melt, and one in the 0-40 km depth range, which is probably a magma chamber associated with active volcanoes above; and (iii) a northerly dipping high-velocity zone that bisects the two low-velocity anomalies, which we interpret as an underthrust forearc sliver of continental origin. Based on He³/He⁴ measurements from volcanic outflows, it is possible that the magma supply is contaminated by interaction with this forearc sliver as it ascends from the melt region below.