A new polymorphinid genus and species Allopolymorphina flindersi Quilty and Whitehead is defined from earliest Pliocene shallow-water, fully marine, calcareous sediments from eastern Flinders Island, northeast Tasmania, Tasman Sea. It is a large, elongate form up to 2.5 mm in length (holotype 1.85 mm), initially polymorphinid but changing early to uniserial frondicularine in later chambers. Specimens occur in most samples of this study, but not in others collected previously from the island. The thin source sequence comes from a previously unrecognized depositional event in outcrop in the western margin of the hydrocarbon-bearing Gippsland Basin of southeastern Australia. Deposition occurred during an interval of high sea level and major coastal onlap.