Science Fiction: Introducing... New Sydney

Global warming has forced the abandonment of coastal cities, and Australia's response was to build New Sydney high up on the Escarpments of the Blue Mountains surrounding an artificial salt water sea backed up behind dams. The Sea water is pumped from the ocean to fill the valleys of the Blue Mountains.
The city itself is highly urban with a population density of one hundred thousand per square mile and an over all population of thirty million.

That should give a start to the concept...