Wirrahs (Acanthistius) are carnivorous reef fishes occupying mesopredatory niches in temperate and sub-tropical ecosystems in the Southern Hemisphere. Despite likely playing an important role in structuring local food webs, direct information on their diet is scarce, with most inferences based on the feeding ecology of analogous tropical reef mesopredators such as epinephelids. Here, we assess the diet of Acanthistius using non-invasive X-ray and micro-CT imaging, revealing a substantial dietary component of shelled molluscs in addition to the more typical prey items such as fishes and crustaceans. Dietary observations obtained through these imaging techniques were subsequently validated by dissection of a subset of specimens, allowing direct assessment of the extent to which X-ray and micro-CT imaging underestimated or failed to detect dietary components. In doing so, we evaluate the strengths and limitations of these non-invasive methods and demonstrate their utility for dietary studies of museum specimens and other valuable material where destructive sampling is undesirable.