Continuous seismic profiles have been obtained over 3000 km of traverse across the continental shelf and upper slope of southeast Australia between Sugarloaf Point and Gabo Island. Acoustic basement consists of Lower Palaeozoic or Permo‐Triassic rocks underlying relatively unconsolidated sediments of Tertiary to Holocene age. The profiles show the basement surface sloping eastwards from the coast to the middle or inner part of the shelf where there is either a ridge (offshore Sydney Basin) or a step (offshore from the Lachlan fold belt). This basement ridge or step separates thin or non‐existent sedimentary cover to the west from a sequence to the east thickening seawards. Much larger basement ridges cross the shelf in the Sugar‐loaf Point and Shoalhaven areas. The former strikes southeast, the latter southwest. Major basement depressions occur southeast of Newcastle and off Disaster Bay.