To support the variable generation from wind and solar you don’t need more fossil fuel generation, you just need to keep what you have at the moment.
The times when wind and solar generation cannot meet demand are going to be something like 20 to 40% of the time, and probably in Australia would come outside the daytime availability of solar power. During these gaps wind and solar would probably still provide on average around 50% of the power demand. So the fossil fuel use would be required to provide something like 10 to 20% of the power generation, so CO2 emissions would be down to 10 to 20% of the levels compared with using fossil fuel generation all the time. With significant hydro and other storage you can do better.
Since this coal and gas generation tends to exist already there are no new capital costs, and the fuel costs are approximately going to follow the use. If you have to provide new generation then do it with gas because the capital costs of this are much lower than that of wind and solar, so the backup capital cost probably adds no more than 10-20% of the wind and solar capital cost at the moment. As wind and solar get cheaper this percentage for backup will rise, but the total cost comes down, of course, as does the average cost of electricity generated.
