From elsewhere:

   this applies to NSW, South Australia and part of Queensland.

so NSW and South Australia will be staggered in real time as they are in different time zones.

As for everybody in the same time zone .. they are all seeing the same sun angle at noon (more or less) and all sharing the same over supply of power from all the grid connected solar power rooftops and farms. It's free surplus power during that time frame.

There's too much available power then (curtailment/negative prices are fairly common now on sunny days), and not enough during the evenings, so it's an incentive for those who don't have/can't get batteries (e.g. renters) to shift their habits. It also can be spun as a cost-of-living action.