Between 1951 and 1959 four engineering geophysical surveys were made in the Great Lake North District of Tasmania. Gravity, magnetic, seismic, resistivity, and radiometric methods were used to investigate geological conditions and rock properties along the line proposed for tunnels and other works for a hydroelectric scheme. The surveys provided much information that was of value in both planning and construction of the scheme. This Bulletin incorporates the description of the four surveys and the results obtained by them. During the period of the geophysical surveys more diamond-drill holes were drilled by the Commission. In this Bulletin, the region surveyed will be referred to as the Great Lake North District and the separate parts as the Intake Tunnel Area, the Outlet Portal Area, the Penstock Line Area, and the Tailrace Tunnel Area.