For the cost and expense of building a pumped hydro plant though, you could just deploy batteries which will do the same thing for a much lower capital and management investment and vastly simplified engineering. And a higher round-trip efficiency.

LiFePO4 works to demand shift on a daily cycle just fine and scales better to solar input (where you need much higher power handling so you can charge it on limited sunlight - a pumped hydro system is limited to charging at about half its discharge rate).

I think that's the point about the lake Onslow project - its MASSIVE. So yes, expensive, but months of backup for the whole country would not be cheap even with batteries