Unfortunately, CNM doesn't contain the category molluscs. I found these shells at the little dam among the dam edge vegetation (which includes Eleocharis ? cylindrostachys) ; perhaps Freshwater mussel Velesunio ambiguus, see https://keys.lucidcentral.org/keys/v3/freshwater_molluscs/Freshwater_Oct18/Media/Html/velesunio_ambiguus.htm
I think someone was digging them out (lots of traces of digging at the little dam) and perhaps used them as bait. Unfortunately. There is a lot of digging around the little dam as well as yabbying going on. There are bricks with ropes etc.