I would not declare your pregnancy because technically you do not have a dependent yet! So don't worry about that at all.
I would, however, be careful about the first home buyers grant/being a defacto couple. But like a PP said, if you don't plan on living together, then you're not in a defacto relationship and you don't need to state this. If you don't have shared bank accounts or other shared connections, he is simply just the father of your child and you're not living together - the way I see it you don't need to state that you are in a defacto relationship because you are not!