I thought the other poster's comment was relevant, so I didn't answer (I posted an earlier comment being replied to). FWIW, I instead would have suggested that natural gas could cover any shortages, since this type of power is easy to turn on and off, and fits more with the theme of "this is potentially possible today".

The simulation over produced energy from green source. Looking at the chart - looks like the wind power alone was enough to satisfy the grid if available. Given that, I do think it's possible that a mere 5 hours of backup can keep the grid 98% renewable, turning to hydro when solar + wind is not enough. Also solar / wind may be somewhat inversely correlated and north / south Australia also may have different weather patterns.

Given a large enough sampling of the world, I'd imagine it would be easier to find the wind / solar power you want, although politics could potentially make that harder.