If it looks like an australorp but is tall and slender then it could very well be a rooster. Here is a picture of one of my australorp hens at 5 months old:
At four months old, my australorp rooster was not nearly as heavy and looked like a tall, gangly thing. Either way, they are a stunning breed.
Richard

At four months old, my australorp rooster was not nearly as heavy and looked like a tall, gangly thing. Either way, they are a stunning breed.
Richard