Quite likely they were there before flooding, and have persisted at bottom of billabong since it was last flooded. these plants have tubers, buried in the thick heavy sediment, and that's how they. the tubers are starchy, and valuable traditional food.
its hard to be absolutely certain without close up of the little fruits on the inflorescence or checking the colour of the spike later in the season, but based on numbers of fruits, leaf characteristics and colour of spike, I am treating this as Cycnogeton procerum.