The investigation of sex bias and pseudohermaphroditism is a relatively unexplored area of research in the Neostromboidae. Here, we inform on these phenomena in a population of Gibberulus dekkersi Maxwell, Hernandez Duran, Rowell & Rymer, 2021 on Green Island, Great Barrier Reef. We collected 598 individuals, and noted the size of the cluster in which it occurred. Each individual was placed ventral side up and the body of the animal observed at the time of righting. Sex was determined by the presence or absence of a verge. During this sexing process, females were inspected for the presence of external male sexual organs to indicate the presence or absence of pseudohermaphroditism. We found that, overall, the population was not sexually biased towards one sex. However, we did find that individual clusters within the population showed significant sex bias, with different clusters favouring either males or females. Smaller clusters had a structural sex-ratio bias in favour of females. No evidence for pseudohermaphroditism was recorded, which indicates that the marine pollutant tributyltin (TBT) may not be present or at harmful levels in the surrounding environment. This study adds information from an unstudied Queensland taxon to the growing evidence on sex bias, clustering effects and pseudohermaphroditism within the Neostromboidae.