Well its always bloody dry on the Nullabor of course. Climate wise there is a big difference between the north (Broome, Darwin, Cairns, Brisbane) and the south (Perth, Adelaide, Melbourne, Hobart, Sydney). The middle is HOT.
In the north you want to avoid The Wet - late October- April/early May.
In the south it gets pretty warm in January/early Feb. Christmas & New year aren't such a show stopper as in the UK but school hols in January can make it more expensive. The Australian winter can be cool and damp- even in Adelaide - often cloudy and showers, maybe 12C-15C. Colder and wetter (these are relative terms) in Melbourne and Hobart but very pleasant in summer.
TBH they're set up for the heat and it's also dry its not as uncomfortable as you'd think (tho Darwin, Cairns and even Brisbane can be steamy). October- November (spring) is probably best for me in SYD, ADL, MEL.
If you think S Spain or S Italy that's the sort of climate you get - but drier