If I understood correctly, they are using batteries in same manner as pumped storage:

https://www.energy.gov/eere/water/pumped-storage-hydropower

We have a few of those for decades, it is actually quite simple, when there is surplus of el. energy it is used to pump water on a hill and acumulate it there. When you need it, the water is than used to generate el. energy. This is the traditional way of flattening the spikes in el. grid.

Now I wonder why the batteries were choosen instead of water, are they cheaper? Does it acumulate more energy on less space than water, it is due to efficiency (there is some loss when you pump water up) or for Austrialia, the water based storage is just not an option (not enough water / hills)?