In terms of what the options are, there's basically 4 that could plausibly be built (though two of them are pretty unlikely). The figures quoted come from AEMO calculations not mine.
1. Wind, solar, batteries, short duration pumped storage + 19 GW of deep firming capacity, comprising 9GW of existing hydro and Snowy 2.0 and 10GW of intermittently operated fossil fuel plant (in practice gas / diesel).
2. As above but replacing part or all of the gas / diesel plant with more large storage hydro.
3. Nuclear power as a partial or even full alternative to all the above except the existing hydro which would be retained.
4. As per (3) but using coal instead of nuclear. This would represent the abandonment of addressing the climate change issue but technically it's possible so I've included it.
Noting that a combination of approaches is possible. Eg build a bit more hydro to reduce but not eliminate the gas / diesel. Or build a single nuclear power station to reduce the scale of the rest but not eliminate it. Any combination is possible, it's not an "all or nothing" situation since they all produce the same output, they all generate electricity.
That said, realistically, option 1 with perhaps some bits of option 2 is by far the most likely under present circumstances and is the base case scenario. So that is, we build wind, solar, shallow storage (batteries + short duration pumped hydro) and we retain all existing hydro. For long duration storage we
might add some more hydro but not to the point of eliminating gas / diesel which does the rest.
Reason to not simply use wind, solar and shallow storage comes down to the problem of multiple consecutive days of poor wind and solar yield. Some will dispute it but suffice to say BOM data, and the real world operating experience of wind and solar installed thus far, both clearly show it to be a problem. That's how AEMO came up with needing 19 GW between large storage hydro and fossil fuels, that's the specific reason for it.
Hydrogen? At some point sure but it's not a commercial off the shelf technology right now and we're unlikely to see a pure hydrogen facility running in the near term. It's still some way off from being a real alternative.
Gas? Beyond a peaking and backup role the basic problem there comes down to a question - what gas? Once we take into account residential, commercial and industrial use of gas "as gas" (that is, not for electricity generation) and we add in committed exports then to be blunt we ain't got no gas. At least not from existing known reserves. It works otherwise but that's the reason it's not seriously on the table as an option beyond a peaking and backup role. Same with oil, it's an imported and fairly costly resource in practice.