My employers the same as Mrs Mac. I had to sign a stat dec saying that I won't earn any money while on mat leave. I am employed full time and its my understanding that if I decide I only want to work part time, I tell them the amount of hours and they are required to find a position for me.
I'd check with the LHMU or AEU Ausgirl, depending on which award/ agreement you come under. Just be aware that the Early Childhood Agreement is currently being renegotiated and won't be released till April.