Blechnum durum in the hard fern genus is within the family Blechnaceae. This fern has a somewhat restricted range in coastal forests from south of Okuru and the Haast River eastward to the Catlins area of the South Island, New Zealand. B. durum also is found on Stewart Island in the mixed hardwood/podocarp forest floor in association with Blechnum blechnoides, Blechnum chambersii and Blechnum colensoi.