US size 20 in women's dresses is UK size 24. This is equivalent to the XXL international size. This dress size has a bust of 120-125 cm (47-49 inches), a waist of 103-108 cm (40-42.5 inches), and a hips size of 123-128 cm (48-50 inches).
So a 24 is equivalent to a size 0.
Your AU size is simply the two numbers of your denim waist size added together. This applies to waist sizes 24-29. So, if you're a size 24 waist, you'll be an AUS size 6 because 2+4=6.
Australian sizes are measured the same as the UK. So if you're a 10 in Australia, you'll need a 10 from a UK-based store. If you're purchasing from an American store, you'll need to go 2 sizes smaller. So if you're a 10 in Australia, you're a 6 in the US.
US size 22 in women's dresses is UK size 26. This is equivalent to the XXL international size. This dress size has a bust of 126-131 cm (49-51.5 inches), a waist of 109-114 cm (42.5-45 inches), and a hips size of 129-134 cm (50-53 inches).
Many people don't realize that Australia is really large, in-fact its about the same size as continental USA.
Fit models for plus are generally a size 2X or 18/20, meaning that this fit model definitely has a curvier figure. Because curves are taken into account in in-style plus size clothing, you will notice a difference in fit.