I think Australorps are one of the hardest breeds to sex. I bought six "pullets" the first week of March and by week three I was pretty convinced that two were roos. I asked on here, used the guidelines (big feet and legs, upright stance, early growth and reddening of comb and wattles) and looked at many pictures of both sex and remained convinced that at least one, which was a great flock protector, was a roo. All that was missing at 20 weeks was the hackle and saddle feathers of a roo. Now that they are laying eggs, none of them are roos. All six are laying and the very roo-acting one is just the Top Hen.
Six weeks ago I bought straight run barred rocks, buff orpingtons and speckled Sussex. Since they were two weeks old there has been no doubt which are roos and which are pullets in the BR and SS groups, and only little doubt in the BO group, mainly because the BO's are growing much more slowly for some reason.
Sorry this is no help, but I thought I should tell you my experience.