alt Hacker NewsFrom the article; this applies to NSW, South Australia and part of Queensland.
So yeah, not universal yet. But the precedent means it's moving in that direction. If WA homes end up producing lots of solar at midday then this opens the door there as well.
It applies in Victoria as well.
Victoria deregulated its market before NSW/SA/Qld.
All of the eastern states (SA/QLD/NSW/VIC/Tas) are part of a single market, with interconnects and wholesale prices set every 5 minutes.
Victoria has its own "default offer" and regulator of the retail market, which is also offering similar "free power" hours.
WA could be part of the NEM with some HVDC across the Nullabor, not sure if it would be economically worthwhile though.