Ghost water once existed but does not any more. It is of particular significance in the federal government buyback scheme for the Murray/Darling where a substantial amount of money can be paid on water rights in the expectation of that water being returned to the river system rather than used in irrigation, but the end result is just a trickle because the water, for which the rights have been purchased, has long gone. Overuse and drought have dried up the river but the ghost water is still part of everyone's calculations.
Our environment is increasingly full of ghosts of what once was. Terms to do with drought and water have resonance in Australian English.