Australia, excluding Western Australia as we are on a separate electricity grid.
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.
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.
Part of the motive of moving the WEM to 5 minute intervals was to eventually leave this option open.
The largest renewable project in the world is being planned in this area[0] so it's feasible that it all may be connected one day