Let me repeat then, probably the largest factor causing coal generation to become uneconomic is its inability to turn off quickly when supply increase and demand stays the same - e.g. when the wind picks up or the sun comes out. Can I have my new brain now?
Negative pricing is a signal from the Market that some supply has to come out of the system. In an electricity grid, supply must match demand. There are ways of increasing demand, such as filling up SA's batteries, or Snowy Hydro pumping water back uphill into their top reservoirs, and to a minor extent fiddling with supply voltage, but there is only so much that can be done. Apart from that, some supply has to be turned off. AEMO does not normally intervene and order generators to shut down, they leave it up to market forces until it gets really serious. And that's what market forces look like.
Also from another page on AEMO's website, this is a good indication of how much grid scale renewables vary up and down. Generally, in the Eastern States grid, black and brown coal is still generating more than 50% of demand, averaged out across longer periods.
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