An Australorp is an excellent laying hen, so overall they are not particularly a broody type (otherwise they wouldn't have the egg numbers they are clocked for per year).
So it is possible your Australorp will never go broody. Commercial lines have been selected for laying over generations, so brooding (which isn't laying) is selected against.
Not that a commercial line Australorp won't go broody. It does happen.
If so, there is no rhyme or reason (literally...a broody hen has very little reasoning, just hormones). Usually the larger breeds go broody in spring if they go broody. Some breeds are notorious for being frequent brooders, and I've had Silkies go broody at 4 months of age. My Cochins, I think were 8 months.
As to incubating eggs, you can do that any time. I would give your eggs a bit more time to stabilize before setting as they are new pullet eggs. Pullet eggs will get larger as they grow a bit older. Don't set very small or very large eggs.
It only takes about 2 days after exposure to a rooster for the eggs to begin to be fertile. One mating will last a hen for at least 2 weeks, sometimes longer (breeders isolate a hen for 30 days), but after that they need to mate again to remain having fertile eggs. (Which the roos are happy to accommodate, regularly).
So in answer to your question, there is no way of knowing if you got the luck of the draw and have a broody type Australorp. If you do, she will likely go broody in spring, although she could sooner.
You will know she is ready to set eggs because she will find a nice nest and sit there 24/7, puffing up and hissing at anyone who dares to look in her direction. She will get up once a day to eat/drink and poo one ginormous poo (the broody poo) that is especially stinky, then she will go right back to the nest to stare off into space in a trance (puffing again at anyone who dares to come near her).
If you do get a brooding hen, it is best to isolate her in her own nice, dark, nest, so that others don't tromp on the developing eggs or push her off the nest. It is best too that she has easy access to and from that nest, and only that nest, so she doesn't get confused and go sit somewhere else.
If you get to that point, there is an excellent Broody Hen thread on BYC. Come on over and join the fun
LofMc