Genetic structure of pelagic obligate freshwater fish in South-East Queensland does not appear to follow any of the of the anticipated models of distribution previously published. A possible model for free swimming fish is the Stream Hierarchy Model with each catchment having a unique population due to the isolation of the catchments over time. In this project's focus on South-East Queensland, there appears to be a mixture of populations with no readily identifiable structure. This project endeavours to explain the current structure using the cytochrome B molecular marker of the Crimson Spot Rainbowfish (Melanotaenia duboulayi). The Bayesian modelling program DIYABC is used to test various hypothesis models. These models were developed from personal experience, anecdotal information, and a review of the available literature. It was hypothesised that an inland dispersal occurred some 300,000 years ago coinciding with the Brigooda volcanic activity. The main hypothesis model predicted an inland dispersal resulting in sympatric populations at a species level, and displacement of one population by the descendant population. [...]