Knowledge of the population structure and feeding capabilities of herbivores is criticalto evaluate their influence on energy flow and community structure in their habitats. We tested forpatterns in abundance (24 reefs) and size (12 reefs) of turbinid gastropods across 4 locations spanning>6° latitude (~1000 km) in Western Australia, and we tested the effect of density (1 to 5 individuals)and size (38.8 to 747.1 g wet wt) of Turbo torquatus on consumption of macroalgae with different thallusstructure (Functional Groups 3 to 5). Turbinid gastropods were found at all locations (up to 2.4 ind.m–2); 82.4% of all individuals were T. torquatus. One location (Marmion, Perth) had considerablyhigher abundances than all other locations. Populations of T. torquatus at the 2 southern locationshad a broad range of sizes (9 to 119 mm total shell length [TSL]), although 1 location was dominatedby small, and the other, by large, individuals. In contrast, both northern locations were strongly dominatedby 1 size class (40 to 60 mm TSL). T. torquatus consumed Ulva lactuca, Hypnea valentiae,Hennedya crispa and juvenile Ecklonia radiata at rates of 150 to 450 mg blotted fresh weight d–1,depending on density of gastropods and species of algae. There was a positive relationship betweenthe rate of consumption and size of T. torquatus when fed U. lactuca and H. valentiae. The presentstudy has produced 3 main insights: (1) densities of turbinids on offshore, wave-exposed, subtidalreefs are similar to those in other coastal habitats; (2) patterns of abundances and sizes are consistentwith broad-scale processes, such as ocean climate, fishing pressure and eutrophication; and (3) T.torquatus can consume a variety of macroalgae at rates that suggest it has the potential to exert topdowncontrol of macroalgae, although low densities of gastropods preclude strong effects.