Bizarre time to have your AOS2 SAC...
I think you should structure the essay like this:
- Introductory paragraph, outlining the history of the Constitutionally entrenched bill of rights in Australia (i.e. previous proposals to introduce one?, Groups and people who have attempted to introduce A B.O.R. etc). Also include some examples of a few countries who've adopted a Charter of Human Rights in the world such as Canada, and explain the positive and negative effects it has had there.
- Reasons for the B.O.R. e.g. discuss how it will establish a really clear outline of the rights we have in Australia, not spread all over the shop like the status quo as Australia heavily depends on Legislation to protect rights. Here, also talk about our Victorian Charter of Rights and Freedoms and compare our protection to the rest of Australia, concluding if we are given better or worse protection.
-Reasons against the B.O.R. discuss why there should not be a B.O.R. in Australia, e.g. we may not require it?
-Conclusion, your stance on the issue perhaps, and whether, like you said, any proposed Charter should have legal Authority and whether there should be a middle ground included in the B.O.R. on which the Constitution may be overridden by the Parliament, e.g. the not withstanding clause in the Canadian Charter
Hope that helps a little bit, my knowledge of AOS2 is a bit sketchy atm, I'm sure you could add heaps more to my advice

Best of luck buddy