Like PP, I also work in HR at a University! smile
This may depend on where you are located. I am in NZ and by law, sick leave does NOT have to accrue from one year to the next. Some employers choose to allow it, but by law they do not have to, and so it then comes down to being a contractual issue. I do not know what the law is in Australia and I guess it may even vary by state.
I would advise that you check your employment contract, and any policies that your organisation might have relating to leave. It should be spelled out clearly, but if it isn't, feel free to ask your employer to clarify.
Also, regarding out the paying out of annual leave, this is quite common and I don't really have a good enough understanding of payroll, compensation, etc processes to be able to explain it properly (or even understand it fully!). But it is something to do with your payment for annual leave being related to how much you've earned, and if you've been at work then you earn more than if you're on maternity leave, so it works out much better for you if you take it when you've been working full-time rather than after maternity leave. Like I said I don't really understand it, but it is certainly a common practice.