A study is justified by the relevant authorities to see how much water is really used as human consumption and how much is used to flush toilets.
The time will come when cities around the coast will have a secondary reticulation system using seawater.
My guess is that it will buy us time before the catchment water runs out completely.
Inland Metros can recycle grey water in a secondary system only for toilets and perhaps gardens. I believe the costs of this will be much less than building more dams.
There are only so many areas which can support dams anyhow.