I did 2 letters.
I gave this one a couple of months ago because they wanted 3 months notice of whether I was taking leave or resigning.

Dear --------

This letter is to advise you that I am pregnant and I intend to take maternity leave.

I will advise at a later time of the date I wish to commence my maternity leave.

Yours sincerely

blah blah blah

A couple of weeks ago I gave another one which basically said something like:

Dear -------

This letter is to advise you I am pregnant, and my baby is due around 11/03/07. I wish to take maternity leave from 16/02/07 for a period of approximately 6 months.

A letter from my doctor confirming my pregnancy and the baby's estimiated due date is attached.

yours sincerely

....

I stole these from the Industrial Relations website, and slightly reworded it.

My work wasnt too worried about it. All they really wanted was a letter to tell them the date I was leaving, and also to give them an idea of when I want to come back, and bubs due date of course. They also have a rule that I had to start leave 6 weeks prior to bubs due date (I think it is something to do with Workcover), but could work later if I had a medical certificate saying I was fit to do so, so I also had to get one of those and give it in to them also. They wanted a medical certificate saying I am pregnant anyway, whether or not I am working within the six weeks. I got a midwife to do this at one of my appointments.