Rach I'm pretty sure it's 10 weeks notice (10 weeks before your due date)..I think...was looking it up the other day cause I need to tell work that I am pregnant!

...from the NSW Office of Industrial relations (may be different in your state)-
"6. How much notice should I give my employer of my intention to take parental leave?

Maternity and Extended Paternity or Partner Leave

* For maternity leave and extended paternity or partner leave, you should give your employer at least ten weeks' notice in writing of your intention to take the leave; then
* at least 4 weeks' before taking the leave, you should give your employer written notice of the dates on which you propose to start and end the period of leave. "

So maybe it's 10 weeks before you intend to start the leave...not sure, I know that in my workplace's leave policies it's a bit more specific.
At the moment I'm planning on leaving at the beginning of June, want to leave as late as possible as I while I can apparently get 12 months maternity leave, I won't be eligible to get maternity leave payments, so need the money while I can get it! So that will be about 4/5 weeks before my EDD, hopefully bubs doesn't decide to be super early! I have an office job though where I'm sitting in front of a computer most of the time, with a bit of walking around the office, so nothing too physically demanding or stressful. Being my first pregnancy I'm not really sure what to expect, maybe leaving that late is leaving it too late and I'll be really struggling by then? Who knows!