Longspined sea urchins (Centrostephanus rodgersii) form extensive urchin barrens in south-eastern Australia, threatening biodiversity and lucrative fishery stocks. Although large urchins are readily visible on reefs, small or ‘undersized’ urchins have often been considered non-emergent, cryptic, and largely inaccessible to predators like the southern rock lobster (Jasus edwardsii). Small lobsters have therefore been considered not to contribute to top–down urchin control. However, evidence from our multi-method dietary analyses suggests that small lobsters can and do consume urchins in the wild and in captivity. Using timelapse footage we investigated further to determine the nocturnal movement of urchins in different size classes. Small urchins emerge from cryptic habitats and are active overnight on open reef areas, providing a window of opportunity for nocturnal predators, including small lobsters, to predate on the urchin. The implications of this work, in conjunction with dietary analysis results suggest that lobsters of all sizes do predate on the range-extending longspined sea urchin and therefore can contribute to top-down predatory control. However, spatial analysis implies that this control method is likely to be more effective in habitats where small urchins are common.