Outbreaks of coral predators are defined as increases (often rapid) in their abundance above threshold densities that can be sustained by local coral assemblages, which in turn depends on the abundance and turnover of coral prey. To investigate the outbreak densities of the corallivorous gastropod Drupella cornus, we conducted both in situ feeding and coral growth experiments at Mandu reef within the Ningaloo Marine Park, Western Australia. Over two 10-day periods, we tagged and photographed feeding scars on colonies of the tabulate coral Acropora spicifera that harboured Drupella feeding aggregations. We calculated a mean in situ Drupella consumption rate of 1.16 ± 1.1 cm2 coral area individual−1 d−1. We also determined coral growth rates by tagging and photographing 24 colonies of Acropora spicifera at time zero and then again 1 year later. We calculated a mean linear extension rate of 7.9 ± 3.7 cm yr−1 for actively growing Acropora spicifera, which we then used to estimate A. spicifera growth rates over a range of coral cover values. This combination allowed us to determine the maximum number of Drupella that could be sustained across a range of coral cover. Our data suggest that the outbreak density of Drupella at the average level of coral cover for back reef sites at Mandu reef (17.6 ± 13.7%) is approximately 0.95 individuals m−2 reef area. At the maximum coral cover observed at Mandu reef (60%), the outbreak density of Drupella is estimated to be approximately 2.83 individuals m−2 reef area. Establishing Drupella outbreak densities assists managers in predicting possible outbreak abundances and in monitoring coral reef health.