It may sound like a strange question, but to a certain extent it depends on who your employer is. If your employer is a Pty Ltd Company then you are most likely covered by the Federal Laws, if your company isn't you will most likely be covered by State laws. I see you are in SA, so give your Department or Employment (or equivalent) a call just to check out the process. As a general rule, you need to give notice, about 10 weeks and generally you can't take leave more than 6 weeks before the birth (the legal term is date of confinement). That is unless you have a medical certificate saying that you should stop. Of course, it also totally depends on your boss who may be happier for you to start leave earlier, but no matter when you begin your leave, it is 52 weeks from the date of commencing leave.

Hope it helps.

Spring.