Nitella sonderi A.Braun in family Characeae was described in 1852, on the basis of a single specimen collected by Ferdinand von Mueller from Melbourne. This specimen was lost and not referred to again, nor illustrated. From 1857 until 1935, three other specimens of N. sonderi were examined or referred to in published literature. The species was amalgamated with the Asian species N. pseudoflabellata in 1962 and a neotype (Lewis 6) was assigned in 1972. In the year 2000, a specimen with the same collection locality and collector as the type of N. sonderi, labelled N. sonderi, was found in the National Herbarium of Victoria (Mueller, September 1852). In the present paper, I deal with the following two issues: typification of N. sonderi (i.e. is the Lewis or the Mueller specimen the most appropriate type?) and whether N. sonderi and N. pseudoflabellata are the same species. The Lewis and the Mueller specimens were compared with the protologue, and the neotype is retained. Oospores and vegetative features of N. sonderi were compared with type material of N. pseudoflabellata var. imperialis (sensu R.D.Wood) and N. sonderi is reinstated as a separate species on the basis of its vegetative morphology, dioecy and oospore morphology, and a description is given with reference to specimens from its entire range. N. sonderi is a spring and summer-growing annual of sheltered sites in temporary wetlands throughout Victoria, New South Wales, and upland south-eastern Queensland. It persists during dry times via a bank of dormant, long-lived oospores in the seed bank. Linking a good taxonomy with thorough ecological studies in this way can inform management of water resources.