Among species, relative brain size (RBS) is correlated with aspects of ecology and behaviour. We analysed patterns of RBS in Australian birds based on 3164 measurements of brain size in 504 species, and provide species-level data for further analysis. Regression slopes calculated both with and without phylogenetic correction are provided for all species and for well-represented orders and passerine families. Patterns of brain-size allometry differ among orders but the evidence for variation among passerine families is equivocal, depending on the method of analysis. These differences are attributable both to absolute differences in RBS corresponding to different regression intercepts and to different regression slopes. Allometric patterns in Australian birds are virtually identical to those reported elsewhere, with large RBS in parrots, cockatoos and owls, and particularly small RBS in galliforms, dromaiids, grebes, swifts and swallows. Our data can be used to generate hypotheses about the drivers of RBS in particular avian groups. For example, small RBS in unrelated aerial foragers suggests that physical constraints may influence the evolution of RBS.