Predators structure ecosystems by limiting prey populations. However, when predation pressure becomes reduced or absent, prey can become overabundant, and the resource requirements of large prey populations can collapse ecosystems. This ecological model applies to some spiny lobsters (Family: Palinuridae) predation of urchins. Large lobsters (>140 mm Carapace Length) are considered key predators due to their apparent increased capability to eat urchins. Recently, the tendency of the largest spiny lobster Sagmariasus verreauxi to eat the controversial large urchin Centrostephanus rodgersii on the SE coast of Australia has been questioned. To investigate whether C. rodgersii are key S. verreauxi prey, we used Bayesian isotope mixing models analyzing δ¹³C and δ¹⁵N values from 275 lobsters collected** over 1,050 km of coastline. Our results contradict the commonly proposed model of these lobsters as major C. rodgersii predators in SE Australia, as our data indicate S. verreauxi derive their nutrition mostly from diverse invertebrates like mussels, snails and other** urchins (43.0 ± 11.9 % mean nutritional contribution ± standard deviation) and surprisingly, brown macroalgae (36.5 ± 8.7 %) rather than higher trophic level prey like C. rodgersii (as its grouping had low levels of potential contribution: 12.4 ± 4.6 %). We detected only a marginal increase (4.5 %) in the potential dietary contribution of the C. rodgersii group in small (< 120 mm CL) compared to large (140–180 mm CL) and very large (165–180 mm CL) lobsters. However, this may also indicate feeding upon other high trophic level sources within the C. rodgersii group; the ascidian Pyura praeputalis or crab Guinisea chabrus. The contribution of the C. rodgersii group** decreased from the North Coast (38.4 ± 16.6 %) compared to Eurobodalla regions (16.0 ± 6.4 %) and generally from North to South (6.4–17.5 %). Our findings indicate that S. verreauxi nutrition depends primarily on invertebrates other than C. rodgersii, and macroalgae. Hence, their perceived ecosystem role as key C. rodgersii predators appears unlikely. However, we cannot comment on the possible diet of much larger lobsters > 180 mm CL that were known to be common on these subtidal reefs many decades ago.