With that reddish brown in the neck and those light slate legs, it's definitely not a Black Australorp. It looks like it may be a mixed breed rooster, perhaps a Black Copper Marans crossed with some other breed.
We thought Australorp as well. But they are usually solid black with the irredecent colors, but not usually red. I have an Australorp hen and he is slightly taller than her but not thicker. She has no red in her at all. I was thinking maybe maran/australorp mix? Or maybe australorp/rir?