Although described as a distinct species in 1869, for more than a century now New Zealand's grey-faced petrel (Pterodroma macroptera gouldi Hutton, 1869) has been regarded as a subspecies of the great-winged petrel (P. macroptera A. Smith, 1840). However, several authors have recently questioned whether the taxon once again deserves full species status. Here, we demonstrate that the grey-faced petrel is sufficiently distinct in multiple facets of its biology (including mitochondrial DNA, plumage variation, morphometrics, osteology, vocalizations, external parasites, and feeding and breeding biology) to warrant reinstatement of full species status under the morphological, phylogenetic, and potentially biological species concepts. Moreover, we present new evidence from mitochondrial DNA (cytochrome c oxidase subunit 1 and cytochrome b) that suggests the great-winged petrel is actually more closely related to the white-headed petrel (Pterodroma lessonii Garnot, 1826) than to the grey-faced petrel. The reclassification of grey-faced petrel to full species status raises the degree of seabird endemism in New Zealand to 43%, emphasising the status of the archipelago as a hotspot for seabird diversity.