I am all in favour of building another dam in Sydney. One slight problem though, you need water to fill it. If the dry pattern continues building another 1 or 100 dams is not going to fix the problem.
Plenty of water in the sea lets use it and power it with a nuclear plant. Plenty of water goes down the drain, lets re-use it and pump it back upstream.
Just a note, the guy who comes up with a way to dispose of sewerage without using water, cleanly and economically should become a trillionaire.