Bun - a friend of mine had a very similar issue - she let slip to her boss the she contacted centrelink and that they will check with the employer why he wouldn't extend her leave - guess what he did? Packed himself!!! Gave her a bonus and let her finish the date she intended. I don't think it sounds right that your boss is trying to make you finish when its convient for him - the law is the law and he needs to obide by it.
Contact your HR dept (unless of course you are it!) and im pretty sure they will tell you that its not acceptable.
Another friend had just changed jobs when she found out she was pregant lucky they she had worked at previous employment for 9 mths and new one for 3 when she applied for PPL. She wasn't entitled to the company Mat leave but she was for the PPL
You could always get the baby bonus - similar thing isn't it?