I do not think it is Potamogeton, which has submergered stems, and the submerged leaves of which are narrow and translucent. I do not recognise it at all. Perhaps something from dumped garden waste.
I felt Potamogeton was a bit of a long shot, I wish it had visible flowers. It has some similarity to some images online of floating primrose willow (Ludwigia peploides) but not to any of the Ludwigia images on CNM. One image (p38, top right) of “Recognising Water Weeds - Plant Identification Guide“ https://www.dpi.nsw.gov.au/__data/assets/pdf_file/0007/329308/041209-DPI-RWW-PLANT-GUIDE.pdf for Ludwigia peploides subsp. montevidensis looks a bit like it but this image also looks different to other examples of this species, otherwise this would have been my alternative suggestion.
I agree this is not Potamogeton and agree it is not quite like Ludwigia peploides subsp. montevidensis. However it does look like Ludwigia peploides, see: https://www.arkive.org/creeping-water-primrose/ludwigia-peploides/image-G142368.html https://www.swcd.net/invasive-noxious-weeds/water-primrose-aka-ludwigia/ http://www.backyardnature.net/n/h/primwill.htm http://www.freenatureimages.eu/plants/Flora%20J-N/Ludwigia%20peploides/index.html#Ludwigia%20peploides%201%2C%20Saxifraga-Rutger%20Barendse.jpg
Michael’s link caused me to check on Ludwigia more carefully. Vicflora states that the leaves can rarely be orbicular. The CABI site states that early growth resembeles a rosette of rounded leaves growing on the water surface, with alternate leaves later. https://www.cabi.org/isc/datasheet/31673 There is a photo of Ludwigia peploides subsp. peploides in EOL showing this character, which is not mentioned in any of the Australian sites for subsp. montevidensis. https://www.cabi.org/isc/datasheet/31673 However, there is no evidence of lanceolate leaves in this photo.
Stephen Skinner of the NSW Botanic Gardens said that the phot doesn't contain enough information to determine which Ludwigia it is but…1. L. peploides var montevidensis is the one that is reported to be local, and was the one I frequently encountered; 2. Without flowers they are often difficult to separate…L. palustris has sessile flowers in the axils of the leaves, L. peruviana has shortly petiolate flowers in the axils of the leaves and L. peploides has flowers on long petioles in the axils of the leaves; and 3. L. peploides is native, the other two are notifiable weeds.
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