Late Permian and Triassic successions of the Sydney Basin, exposed in cliffs of the north coast, south coast and Blue Mountains, include sandstones in which quartz content increases up‐sequence and palaeocurrents swing from southwesterly through southeasterly to northeasterly. These basin‐wide phenomena are interpreted to have resulted from the northeasterly migration of a drainage net, in which northeasterly‐flowing tributaries bearing quartz sand from the craton met southwesterly‐flowing tributaries bearing labile sediment from an arcuate rim‐orogen, and blended in a southeasterly‐flowing trunk stream down the axis of a foredeep. Migration of the drainage net records the retreat of an arc‐derived clastic wedge in the latter part of a megacycle which extends from the top of the mid‐Permian Nowra Sandstone to the top of the mid‐Triassic Hawkesbury Sandstone. The same megacycle occurs in the Bowen Basin, and may also occur in the Nilsen‐Mackay Basin of Antarctica.