Gas is tied up largely in export contracts, and the whole thing about Australia being both somewhat flat and dry means hydro basically doesn't exist (it exists only in the literally 1 place on the mainland continent we have any "significant" snowfall).

The biggest surprise to me was that a plant in Gladstone (Queensland) was contracted to prop up a plant in Victoria. These two places are 1375 miles apart. Then again, I guess the linky bits between the wires must be pretty high-speed, so the speed-of-light propagation of the grid conditions wouldn't take too much extra time.

You're confusing coal plants as flywheels and coal plants generating power -- two different things, two different timescales.