C. mayhillae has a shell that measures up to by, with a subdiscoidal shell of four whorls that increase in size. The shell's spire is flat, and the protoconch has 1.75 whorls with the first and last quarterwhorl having nine spiral lirae. The shells are uniformly brown in colour.
C. mayhillae is endemic to New Zealand, found in inland and coastal forested areas of the Northland Region. The species is typically found in inland forested locations, in association with the bark of podocarp trees.