There was a clump on the damp part of the south shore hidden in grass/sedge. The fact that the marker is in the middle of the dam is GPS inaccuracy- will fix
I can't see the location (which dam it is) until the plant is identified, but it looks like the Myriophyllum simulans planted on Isaacs Ridge (Mugga Lane) so it could have been part of the same project.
I have tried both the VicFlora and PlantNET keys and both come down to Myriophyllum simulans (emergent leaves in whorls of mostly 3 or 4, sometimes irregularly so; stigmas pink, fading to white) or Myiophyllum variifolium (emergent leaves in strict whorls of usually 5; stigmas white). It does not really fit either.