Gahnia filum, the chaffy saw-sedge, is a tussock-forming perennial in the family Cyperaceae, endemic to Australia. It grows to between 60 and 110 cm in height. The species occurs in coastal salt marsh in the states of New South Wales, Victoria, Tasmania and South Australia. The caterpillar of the Altona Skipper butterfly uses this species for shelter during daylight hours, binding the leaves with silk. By night it feeds on the grass.