Ann: Noosa Mining Conference Presentation, page-15

That's a great resource. Google maps is always out of date. If you change the date back early June you can definitely see the water level has changed by the amount of rocks showing.
I don't think it will ever be drained completely. I haven't looked up what the planned change in head height is for an 8 hour run but I would expect it to be less than a few metres. You could probably get a pretty close estimate using the surface area of the reservoir and the flow rate of the pump.The aim of hydro is to keep the head pressure (water height) the same for as long as possible.