Biogeographic breaks are locations where multiple species reach range limits simultaneously. Numerous breaks have been identified; however, the mechanisms that generate and maintain these breaks are largely unresolved. One break of particular interest lies between the subtropical Lord Howe Island off the east coast of Australia and the Great Barrier Reef 1000 km to the north, because of the potential for Lord Howe Island to serve as a refuge from climate change. Our aim was to quantitatively disentangle the mechanisms proposed to explain this break. To do so, we combined species abundance and trait data to test the hypotheses that dispersal limitation, environmental tolerance, competitive ability or evolutionary processes maintain the break. Specifically, we used multiple linear regression to analyse the extent to which species traits could predict the location of species along a non-metric multidimensional scaling axis, which was representative of assemblage change across the break. Three of the twelve species traits considered – reproductive mode, upper depth and depth range – contributed significantly to the averaged model. The higher relative abundance of species that brood larvae on Lord Howe Island supports the hypothesis that local retention of larvae is critical for population establishment at this location. In addition, abundant species on Lord Howe Island were disproportionately associated with global depth distributions that encompass deep water, which might indicate that these species are better able to tolerate sub-optimal environmental conditions, such as low light levels in the sub-tropics. Therefore, we conclude that the biogeographic break between the Great Barrier Reef and Lord Howe Island is maintained primarily by the ability of species to establish populations upon arrival. More broadly, our work adds to a growing body of evidence that suggests, over large temporal scales, establishment is more important than dispersal for the maintenance of biogeographic breaks in the oceans.