The Boulder Bank is an eight-mile (13 kilometers) stretch of boulders in the Nelson Harbour. The geological origins of this unusual natural formation are not well understood, but it was likely caused by longshore drift. Boulders were pulled off nearby headlands and formed the current structure over thousands of years. This unique formation is composed of granodiorite, the same type of rock from Mackay Bluff.

This structure creates a natural harbor, which was later cut to provide access for larger ships entering the region between 1903 to 1905. The section is now known as Haulashore Island.

Boulder Bank is now protected as a scenic reserve and is the nesting site for several bird species.