15cm long by 2mm or 3mm wide, black coloured worm. Seen moving through algal deposit on the bottom of one of the feeder creeks for Bogong Creek (WaterWatch Bogong Creek Central sample site)
Thanks @trevorpreston . It was a long shot. Just that we've been sampling at that site since 2016 and never seen an aquatic invertebrate there of that size.
I have fiddled with the image but unfortunately cannot extract sufficient clarity to venture an ID. From the overall shape and details provided, it could be a flatworm. They can survive under water, though few species of which I am aware would enter water of their own volition.