Australorps go broody fairly easy. And it does should like she is. If she is growling at you and raising her neck feathers when you try to move her out of the box, and when she is out she cluck cluck clucks all over the place, you have a broody. You can let her set or try to break her broodiness. It is entirely up to you.
Kelsie has left you with some great links for some good reading. If you have any other questions, feel free to ask.
Good luck with your chicks and broody and welcome to our flock!