Two-dimensional, lithospheric-scale magnetotelluric imaging in the central Australian Proterozoic has constrained the large-scale architecture of terrane assembly during Paleoproterozoic accretion and collision. The comparatively conductive North Australian craton, consisting of rocks between ~2500 and 1730 Ma in age, has been imaged to extend for 150 km under the 1690–1620 Ma Warumpi Province, which forms part of a large, comparatively juvenile terrane in central-southern Australia. Collision between the North Australia craton and Warumpi Province occurred ca. 1640 Ma. The boundary between these domains is modeled to be subvertical at crustal scale, but dips south at ~45° in the mantle to depths of 150 km. We interpret this geometry to reflect lithospheric-scale underthrusting of the North Australian craton beneath the Warumpi Province, and suggest that it provides a first-order constraint on subduction polarity during collision ca. 1640 Ma. In contrast, most contemporary models for the evolution of Paleoproterozoic Australia propose that the North Australian craton was located on the overriding plate of a long-lived (ca. 1800–1550 Ma) north-directed subduction system. The polarity of these models is not consistent with the lithospheric-scale geophysical architecture.