Codfish Island is the southernmost breeding location for Cook's Petrel (Pterodroma cookii), endemic to the New Zealand archipelago. To provide a population estimate and indication of population growth following introduced predator eradications we conducted an island-wide survey of Codfish Island within two a priori defined strata. Plot surveys revealed only five burrows within forest habitats from 175 plots surveyed and the locations of these and other burrows observed incidentally were used to identify three key areas of Cook's Petrel breeding activity that were subsequently surveyed using line transects. Within these areas, 42 burrows were counted with burrow densities ranging from 0.0003 to 0.002 burrows m−2. Burrow densities in conjunction with the three-dimensional surface areas of the surveyed locations suggested a minimum of 6,194 ± 956 burrows present and equated to approximately 5,000 (95% CI 3,000–6,000) breeding pairs, using a burrow occupancy estimate of 80%. The Cook's Petrel population on Codfish Island appears to have increased markedly since Weka (Gallirallis australis ) and Pacific Rat (Rattus exulans) were eradicated in 1980 and 1998 and is consistent with a recent upwards revision of the species' world population size of approximately 1,300,000 (900,000–1,800,000) individuals. Revised population data, and ongoing protection of Cook's Petrel's major breeding sites may now qualify the species for a revised conservation status moving from Endangered to Vulnerable under IUCN criteria.