Earlier quoted context omitted.
I think it's mainly because of large amount of rooftop solars which are either unable or don't have incentive to curtail, and then there are coal generators already running at their lowest output level.
The grid could, at least in principle, curtail rooftop solar by messing with the frequency. This might be a bad idea.
In Australia, we’re moving towards dynamic operating envelopes for solar and other “distributed energy resources”. That potentially includes EVs and ACs.
The idea is the solar inverter is sent a dynamic stream of operating constraints by the distribution business based on network conditions.
It’s already being piloted in places like South Australia, which has very high solar penetration. Regulations are also moving towards requiring smart meters at all households (already the case in Victoria) and all new inverters be network connected.