Considered a different species for 40 years. The form Swainsoni occurs along all the eastern and northern coast of South Island and all around the southern part of North Island. It differ from the type by a more elongate shape, a less marked shoulder with nodules weaker or absent.

Powell (1979) pointed out that the nodules are laterally compressed in Swainsoni but vertically compressed in arabica type. However this feature is variable / hard to distinguish in many specimens. The pattern of the Swainsoni is significantly paler than one of typical arabica.