Hi Andre - that is more difficult - without getting too technical I mean. You need to look carefully at the junction between the leaf blade and the sheath, on the upper surface and look at the hair distributions on the leaf and also on the leaf sheath: Try this:
Austroderia tubraria (Chatham Islands ONLY) - leaf sheath conspicuously hairy, plants hermaphroditic (so all can set seed),
Austroderia fulvida - leaf blades slightly scabrid, upper leaf blade above ligule without hairs, ligule 1 mm (or less) - North Island (scarce in the northern South Island)
Austroderia splendens - leaf blade slightly scabrid, upper leaf blade above ligule hairy, ligule 3 mm long - FLORA V states rhizomatous but there are two entities in this name, the rhizomatous one is found in active sand the non-rhizomatous one on coastal headlands etc.....Continued