To support decision making on placement of protected areas for Hector's dolphin on New Zealand's South Island west coast, we conducted three aerial surveys documenting the species’ distribution in this area. The first survey was designed to quantify alongshore distribution and abundance, and revealed a patchy distribution with a central zone of high density. Two further surveys, in summer and winter, focused on this central zone to quantify offshore distribution in detail. Dolphin density decreased with increasing distance offshore, with no dolphins sighted more than 6 nmi from the coast or in water deeper than 60 m. There was no significant difference in offshore distribution between summer and winter surveys conducted in 2003 (G= 2.15, df = 5, P= 0.83). Partial Mantel tests showed that dolphin distribution was best explained by distance from the coast, in both summer (rM= 0.088, P= 0.0001) and winter (rM= 0.054, P= 0.0004). Spatial contouring techniques showed that small (ca. 5 km) and medium scale (ca. 50 km) patterns of density in the central zone were remarkably consistent, suggesting year-round residency. Based on these data, the current restrictions on commercial gillnetting protect 60% or less of the dolphin population for 3 mo of the year.