The distribution, mode of occurrence and population densities of extensive colonies of worms, and the morphology of their thick‐walled tubes are described. Interaction between two species occurring in certain strata enables the feeding habits of the tube‐builders to be reconstructed, and they are thus identified as terebellid worms. The worms when alive were less than 5 mm in diameter and had tentacles 27 mm long. Their environmental situation was near the southern edge of an extensive area of marine deposition covering much of the Sydney Basin. The depth was below wave base. The worm populations were buried alive during deposition of sheets of sediment, probably resulting from intermittent sudden flooding events in coastal rivers.