"Kalgoorlie super pit is nearly 600m deep and 3.5km x 1.5km."
wrong - totally wrong --
they are the dimensions apx. - but, the length and width is at the top --
LOOK at the picture - at the bottom it's the size of a dinner plate - exactly where you are calculating your head pressure to
you CANNOT have the hole FULL AND have your generating assets in the bottom of the hole
and, you will NOT get the head pressure at the bottom if the bloody hole is full of water or partially empty - the pressure the weight of the water sees to that - because you have to run your water from your upper reservoir DOWN a pipe - and THROUGH your generator and then OUT into the water again - and it's there that the PRESSURE works against you.
Even if you magically had your hole almost empty - the pressure against it builds as the hole fills
