My advice would be do not lie to centrelink. If you are in a defacto relationship then tell them.
You will get all the necessary forms when you birth your baby. They will include the baby bonus claim form and the forms for family tax benefit.
I'm doubtful that you would qualify for any centrelink payments (you may be entitled to a heath care card) in your present situation.
You can always make an appointment at one of the offices to discuss your situation.