Four species of turbellarian temnocephalan symbionts (Platyhelminthes: Temnocephalida) are reported for the first time from the external surfaces of Cherax quadricarinatus, a freshwater crayfish from northern Australia. Three of these species — Temnocephala rouxii Merton, 1913, Notodactylus handschini (Baer, 1945), and Diceratocephala boschmai Baer, 1953 — are known previously from related crayfish in New Guinea. The newly discovered fourth species, Decadidymus gulosus n. gen., n. sp., has an unusual combination of characters linking it with both the Temnocephalidae and the Scutariellidae. Together the four species possess an array of characters that challenges current concepts of families in the order. D. boschmai has an almost completely ciliated epidermis, a feature otherwise unknown in the order.