Native predators’ ability to control alien prey depends on both their capacity and propensity to consume newly arriving species. The view of spiny lobsters as effective urchin controllers has been substantially revised over recent decades. Southern rock lobsters (Jasus edwardsii) are considered biological control agents for the range‑extending macroalgal grazer, the longspined sea urchin (Centrostephanus rodgersii). However, lobsters preferentially consume native prey such as the shortspined sea urchin (Heliocidaris erythrogramma), which may limit their effectiveness in controlling C. rodgersii. To investigate the behavioural mechanisms underlying lobster prey preference, we conducted four tank‑based feeding experiments involving 95 lobsters. Hypotheses included: (1) physical traits, i.e. longer spine length and larger body size, increase handling difficulty and in-turn reduce lobster predation; (2) distasteful chemical cues contribute to prey rejection; (3) lobster naivete towards the range‑extender influences prey choice. Experiments included presentation of live urchins for predation, removal of urchin exoskeleton to present only urchin roe for consumption and trimming spines of C. rodgersii.Overall, 10% of C. rodgersii offered to lobsters were consumed. Results consistently indicate that spine length and associated handling difficulty are the primary drivers of prey preference, rather than distasteful cues. Lobster predation on small urchins occurred readily, increasing urchin size substantially increased handling time, likely causing lobsters to shift toward more easily captured prey. No avoidance was observed when the spines were removed by removing urchin exoskeletons. Once lobsters are accustomed to the handling of urchins (i.e. familiar, like those in long-term barrens), preference for natives is still present but reduced. Lobsters can provide predatory pressure on urchins, and this will likely increase as lobsters become familiar with the range-extending prey. Results show that while familiarity of native predators to novel prey may increase predation rates, ultimately, physical constraints may still limit predatory control effectiveness.