Abstract Almost one-third of the seabed off the coastline north and south of Sydney comprises a planated bedrock surface, evident from sidescan surveys over the inner continental shelf. In seismic records, this rock surface extends up to 23 km offshore from the sea cliffs along 300 km of the coast. The rock surface dips offshore to as much as 180 m below sea level, where it merges with a major unconformity in the shelf sediment wedge. The surface is eroded into Mesozoic and Palaeozic rocks and is heavily dissected by sediment-filled, palaeo-valley incision and structural jointing. The sediment-fills comprise sand wedges that thicken landwards to form beaches and estuarine flood-tide deltas, respectively, in smaller and larger palaeo-valleys incised to below present sea level. At the base of the cliffs, the planated surface is buried by shelf sand bodies up to 30 m thick in places. The seaward edge of the surface is everywhere buried by the onlapping continental-shelf sediment wedge. The contiguity of the abrasion surface with the unconformity in the shelf sediment wedge suggests that marine planation began in the Mid-Oligocene, indicating time-average rates of gross cliff retreat at about 1 mm a −1 .