It seems that the sudden disruption to the network caused a voltage collapse.
the interconnector to Victoria was shutdown as it was overloaded.
They weren't able to keep the system stable.
I presume that to prevent such an occurrence during an unplanned event then SA would have to be generating much more energy that it needs. What use could any un-needed electricity be put to?
Pumping water somewhere high to drive water turbines?
Splitting water molecules to produce hydrogen?
Powering a white elephant?