Many geophysical images of ancient orogenic belts utilise seismic reflection data but increasingly magnetotelluric (MT) data have been shown to provide insights into orogenic architecture not evident in seismic surveys, particularly through their ability to image steep structures. A 140km long MT survey carried out in Palaeoproterozoic crust in central Australia has imaged a crustal-scale conductivity contrast across a steeply-dipping boundary. The surface location of this contrast coincides with the Redbank Thrust Zone, previously shown from seismic reflection and gravity data to be a moderately-dipping, thick-skinned fault that produces one of the largest continental gravity gradients known on Earth. The steeply-dipping conductivity interface is interpreted to be a strike-slip boundary separating the North Australian Craton from a younger accreted terrane. This result is therefore a first-order geophysical image of a Proterozoic accretion system in Australia. The steep boundary is not evident in the seismic data and highlights the utility of MT in defining fundamental crustal boundaries.