Snowy hydro should be looked at like a giant battery. The electricity used to pump the water will be "excess".
The grid is far from an efficient beast. And large amounts of solar and hydro will add to this.
Coal base load cannot just be turned on and off. It takes days to fire up and fire down such supply. Gas peaker plants which can fire up in 15 - 20 minutes charge excessive amounts per kw to meet the energy needs of the grid. This is why the "big battery" in SA will pay itself off in 2 - 3 years maximum. The snowy hydro has a buisness case that is for sure as it will be a long term asset that will needs of the grid over and over.
In short the snowy hydro catches excess or wasted energy production and then sells it back to the grid. Like a battery.
Its biggest risk is that other forms of storage will come online. For example VRFB
that are also large scale.
(20min delay)