How much energy does it take to pump it back up from 231 meters, compared to traditional methods? That would be 2.25kJ of energy per liter. Normal desalination I believe is 3kJ per thousand liters.
My idea is to not pump it back up but to let it drain down to Australia.
This is the most sensible idea in this comment chain
You can power it by pumping down an equal amount of surface water
If you had a 231 meter dam, with a membrane filter at the bottom, fresh water will come out at atmospheric pressure. It then has to be carried back up to sea level. You could power that pump with another hole in the dam, but then the non-sea side of the dam would constantly be filling up with water. Once it's full, there's no longer any pressure difference to drive a pump.
That wouldn't work because the system relies on having a pressure differential across the membrane in order to function.