Maybe when you see the new hen getting pecked, run in and break it up loudly. Then the hens will possibly remember you running in and they won't do it again. Not sure because my girls don't fight to the point of blood.
Maybe the Australorp thinks she's the top of the pecking order and she doesn't want anyone to replace her spot. Put the new girls in a separate small coop, kind of like a brooder pen but bigger, and then introduce the new girls and your Australorp between a fence. If she tries to fight, let...