Given the difficulty of effective landscape-scale control of introduced predators, fenced areas that exclude them (i.e. ‘mainland islands') can play an important role in conserving threatened mammal species in Australia. Despite this, the effort required to eradicate or remove introduced species from within fenced areas remains poorly quantified. This study was conducted at Mt Gibson Wildlife Sanctuary in semi-arid Western Australia, where a 43 km predator-exclusion fence surrounding 7832 ha was completed in June 2014. The subsequent effort expended in eradicating feral Cat (Felis catus) and removing European Rabbit (Oryctolagus cuniculus) populations was logged daily during 11 months of active population removal and 3.5 months of monitoring. The total effort expended on cat eradication, rabbit removal and monitoring included over 4800 person-hours of work and nearly 67,000 km of driving (of which 6700 km was for spotlighting), and the conduct of over 15,000 trap nights, 2300 km of sand tracking transects, and over 7800 camera-trap nights. Total costs (in 2015 figures) were estimated at just over AUD $390,000, which equates to approximately $50 per hectare. Cage trapping was most efficacious for catching cats, although it took nine months to capture the last individual, which was detected independently by both sand tracking transects and camera traps. This research provides baseline data on the resources required for future eradication and removal projects, particularly those within fenced reserves.