The idea there isn't that you store power but rather that you only run the plant when power is cheap (and then keep a large enough fresh water reservoir to serve demand until the next cheap electricity arrives). Demand management rather than supply management, effectively.

Yeah, the problem is that it's not easy to store very large amounts of water (at least not near where the Adelaide Desalination plant is).

I believe they can pump the water up into the hill, so I guess it's not entirely out of the question.

Decal plant when the power is cheap and well water the rest of the time?