As M robustum is rare to find in the wild, and only in select places, these days bets are good you have not found some.
Try using Google as there is a lot of information to find. I found this on the NZCPN site...
Features
Perennial aquatic herb which if in pools of water is firmly rooted to the bottom. Stems spongy, inflated up to 1.5 m long, emergent portion 300 mm tall, erect to prostrate. Submerged leaves in whorls of (4-)5-7, (15-)20-35 x (4-)6-10 mm finely divided, pectinate with 26-32 pinnae, brown, these diminishing in size toward water surface. Emergent leaves glaucous, tinged red, narrowly ovate to oblong, apex acute, otherwise similar to submerged leaves. Flowers perfect. Sepals 4, ovate to deltoid 0.6-0.8 x 0.5-0.6 mm, petals 4, weakly hooded, 2.5-4 x 1-1.5 mm. Fruits globular to slightly turbinate, 1.5-2 x 2-2.5 mm.
Similar Taxa
Similar to the naturalised M. aquaticum (Vell.) Verdc. From which it differs by being monoecious (males and females on the same plant), the exposed stems usually pink coloured and by the leaf tipes which are acute rather than blunted ended