When you say...
I'd do it like this:
Para1: indicate if you agree or disagree, outline the principle of seperation of powers
Para 2-? : discuss the importance of the other principles of the australian parliamentry system, comment on their importnace (both independently and relative to seperation of powers) and why they are important.
Last Paragraph: Depending on whether or not you agree or disagree, explain your position drawing on the conclusions you made in your previous paragraphs.
Part 2 - how many principles/points would I need to point out/use (knowing that the question is worth 8 marks)?
well... I would do these:
1) Role of the crown (very good for showing an aspect of the system which can be heavily criticised)
2) Responsible Government and Representative government (combined in one paragraph, but make sure to differentiate the two)
3) Roles played by the houses of parliament (and focus on the senate)
I think you can more or less neglect structure of parliament for this question (is that even one of the principles? I forget), but if you think you have time then add it in if you like.