The relative costs are just a question of policy. Legislators could make fossil fuels prohibitively expensive tomorrow if they really wanted. Anyway, if Australia doesn’t have a good source of fissile material (I have no idea), that is a fair point against nuclear power there. However, that just means other big, ambitious emission-free power projects should be considered instead, like deep-well geothermal, concentrated solar, and coerced rooftop solar. Seemingly cost effective half measures that keep fossil fuels in the mix are a mistake.