Very briefly visited, (I'm an Aussie) I saw lakes and dams; but I never really had a look to see if there was a good spot to build a pipe between or to create a new lake.

You don't actually need an excess of water, you need to be able to move enough between lakes though. You are not using any extra water from the system by adding pumps.

That's the real determining factor right, places with cheap big pumped hydro projects ideally already have big dams that are not full, so from that point it works. But then you need a reasonable amount of elevation between reasonably close lakes (or ideal new spots).