Dispersal ability is a fundamental attribute of threatened species that influences their prognosis for survival in fragmented habitats. More vagile species are considered to be under lower threat from habitat fragmentation than more sedentary species. This study investigates morphological variation amongst island and mainland populations of a threatened Australian passerine, the white-fronted chat (Epthianura albifrons), to review previous conclusions about levels of population differentiation and consequent conclusions about dispersal ability. Standard body measurements were taken from wild-caught birds from three islands and three mainland regions of Australia, ranging over a latitudinal distance of 1150 km and a longitudinal distance of 3300 km. Significant size variation in wing, bill and headbill lengths were found amongst populations, revealing a significant latitudinal trend consistent with ‘Bergmann’s Rule’. Even after accounting for the latitudinal trend, island populations showed significant differentiation from mainland populations in some attributes, although island populations were not consistently larger than mainland populations, as predicted by the ‘Island Rule’. A lack of size variation between island and mainland museum specimens has historically been used to conclude that white-fronted chats are capable of crossing an oceanic barrier of at least 30 km. The level of population differentiation identified in this study suggests that previous estimates of dispersal capacity may be overestimated.