White skin is dominant. The yellow skin gene was bred out while the breed was being established, back in the 1920s. If you have an Australorp with yellow skin, something else was bred into the line within the last generation or two. Hatcheries cross breed their 'purebred' birds all the time. That's why the have the disclaimer that their birds don't meet exhibition standards.
They are not hatchery birds, they are from show stock that are pretty amazing quality... I am pretty sure they have white skin, I will have to check in the morning, but they are going to produce some good show babies when I hatch from them this spring...