[1] It is an annual or perennial, diminutive, aquatic herb[2] with elongate, 0.7–1.5 mm wide, erect, sympodial rhizomes with adventitious roots.

[6][7] The male reproductive units, which are fewer in number than the female ones,[2][7] consist of 2–4, lanceolate, 3–5 mm long bracts, as well as 4–6 stamens.

[4] The lectotype was collected by Rodway in Broad River, Tasmania, Australia in December 1896.

[8] The specific epithet filamentosa, from the Latin 'filum' for thread, means thread-like.

[4] It occurs in marshes, pools on mud flats along rivers,[4] lakes, tarns,[2][5] and stream margins.