I have 2, and each time I finished 6weeks prior. Just had a note from the O n G to give your confinement date(sounds very old fashioned). I got 12weeks paid leave and took that at half pay which in effect gave me 6monts paid leave, then I used up my rec/holiday leave at half pay too. I also know people who also use there long service at half pay and get paid the whole 12months! We can also apply to have an extra year off so we could get 2years mat leave.
If you are unwell and have a Dr's Cert to say it is not pregnancy related..I believe you can use your sick leave up until your official Mat Leave and then start Mat leave...had a friend do that as she had pueumonia. You should have a good read of your CA or AWA if you are on one or your HR person. IO also believe they should legally have Mat Leave or any leave or work conditions for that matter written down somewhere for you to view or on a work intranet without having to consult someone too (privacy).

Good Luck

Heather