Nematoceras sulcatum Corybas sulcatus, also known as the grooved helmet-orchid, is one of two helmet orchids endemic to Australia's subantarctic Macquarie Island, and the second to be discovered.

It is a small, terrestrial, tuberous, herbaceous plant that forms clonal colonies.

It occurs on the plateau uplands of the island, growing in wet grassy seepage areas.

It can be distinguished from the only other orchid on the island, Corybas dienemus, by its dark red flowers.

However, the orchid is listed as endangered on the Tasmanian Threatened Species Protection Act 1995 because of its restricted distribution, small population (estimated in 2010 to comprise fewer than 250 mature individual plants), and projected decline through ongoing habitat degradation caused by the grazing, digging and burrowing activities of feral rabbits.