The freshwater gastropod genus Posticobia Iredale, 1943, is found predominately in eastern Australia and is redefined using morphological and anatomical characters. The genus is found in a wide range of habitats from isolated springs in central Australia to the large coastal rivers and lakes of New South Wales and Queensland. The genus as currently recognised is considered to contain three species, Posticobia brazieri (E. A. Smith, 1882) from eastern Australia, Posticobia ponderi n. sp. from Mulligan Springs, South Australia, and Posticobia norfolkensis (Sykes, 1900) from Norfolk Island.