Sexual size dimorphism and geographic variability in external measurements of the Grey-faced Petrel (Pterodroma gouldi) were investigated on the west and east coasts of the North Island of New Zealand. The applicability of morphometrics to distinguish the sex of individuals in the two geographic areas and assign a geographic origin to individuals was evaluated. Low geographic variation in morphometry was found, with discriminant function analysis failing to develop an efficient function to assign a geographic origin (55.9% accuracy). In contrast, sexual size dimorphism was marked (up to 6.6% different), with males being significantly larger than females in all measurements but wing length. A discriminant function combining bill depth at nostrils and head length correctly predicted sex of adult breeders with 80.9% accuracy (78.9% for males, 82.7% for females). Despite its apparent applicability across the entire range of the species, this discriminant function was not sufficiently accurate on its own. Alternatively, the use of bill depth at the base or a bill size index increased sexing accuracy to 92.1% when both mates of pairs were considered. Thus, Grey-faced Petrels can be reliably sexed by taking only one measurement on both mates of a pair.