Tufted, stoloniferous, perennial herb. Leaves basal, to 3 m long, 11–35 mm wide, ± linear, obtuse; margins serrulate. Bracts of male inflorescences 10–25 mm long; flowers <0.5 mm long; perianth segments 3; anthers 2, fused for 75% their length. Female inflorescences 1(–4) flowered; bracts 10–30 mm long, translucent; flowers (1.9–)2.5–4 mm long; outer whorl of 3 green, obtuse segments; inner whorl minute; stigmas 3. Fruit 20–160 mm long. Flowers Nov.– Apr.

LoM, MuM, Wim, GleP, Brid, VVP, VRiv, RobP, MuF, GipP, OtP, Gold, CVU, GGr, DunT, NIS, EGL, EGU, HSF. Occurs in freshwater lakes, rivers and irrigation channels.

In Victoria, this species was previously included in Vallisneria americana Michx. and V. spiralis L. as Australian plants closely resemble these species (Lowden 1982). However, recent molecular studies have found Australian material to be distinct from these species occurring in America and Europe. V. americana is indigenous to the Americas, and in Australia the name was previously misapplied to V. australis. V. spiralis is indigenous to Africa, Europe, western and central Asia, and in Australia the name was also misapplied to V. australis as well as V. nana. Morphologically there are few characters that may distinguish V. australis from V. americana or V. spiralis. Characters previously used to distinguish taxa in Victoria (i.e. features of stamens and the stigma) have been found to be unreliable and or often unobtainable, as such, Victorian plants are now included in a single species.

Some earlier treatments (i.e. Den Hartog 1957; Sainty & Jacobs 1981) included this species in V. gigantea Graebner, a name also misapplied, and possibly a synonym of V. nana R.Br. Vallisneria nana occurs further north, on the east and north coast of Australia. It is distinguished from V. australis by its usually smaller (to 2 m long and 10 mm wide) leaves that are acute, and anther filaments that are free or fused. However, the misapplication of V. gigantea in Victoria and New South Wales to both V. australis and V. nana, along with inability to accurately identify many herbarium specimens further complicates the taxonomy of this group, and it is possible that V. nana may also occur in Victoria.

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