Em, your boss sounds like a twit!! (no offence or anything) They have to offer you your exact same position back. If you were full time before going on maternity leave then they have to offer full time on your return. It sounds as though they are stuffing you around and trying to make it impossible for you. I hate how these things happen! How can they say they never recieved your maternity leave letter? They obviously didnt recieve a resignation from you, and if they called to tell you what was happening, they were obviously still holding your position at the time! It sounds a bit fishy to me, like they are just trying to palm you off and hope you just go away. They got your letter. Theyre just being stupid.

I have also had some issues: I wanted to return part time, which they agreed to. I confirmed it a number of times, and it was even written on my contract, then they changed their mind at the last minute and told me its full time or nothing, knowing full well I could only do part time.

I found that writing a letter helped. I did some research using the Industrial Relations website, and I called the infoline to confirm what I am entitled to. I wrote about 1.5 pages and I sent it to my manager. I also sent a copy to the HR person at the new company (the one that bought the one I was working for.). I basically explained my situation, gave details of all the conversations I had had with my manager (including dates) and basically told them in a very PC way that they were being really unfair to me. I also wrote the advice given to me by the Workplace Infoline and hinted at the fact that what they were doing borders on unlawful. I got a response pretty quickly My manager gave me a call and he sounded very flustered and nervous (very unlike him) so I think I must have made him actually think about it. I wrote the letter because I didnt want them thinking it was ok to do this to people, in the hope that they wont do it to anyone else in the future.
Maybe you could write a letter. That way you (and they) have everything in writing.