I like the start and general idea of your answer, but think it would be stronger if you were more explicit about the High Court's formulation of the protection from the Roach Case. Phrasing it as a negative (ie in terms of what the parliament *cannot* do) can also help, as it parallels the enunciation of our express and implied rights (eg s116 prevents the Cwlth Plt from infringing our freedom of religion by...etc).
So, after your intro, perhaps drawing the chain of logic out a little more clearly - like: a system of rep govt is established by the Const, therefore the Plt cannot prevent significant sections of the people from voting without significant cause (eg prior serious imprisonment) as that would impair the popular representation and democratic quality; the flow-on protection derived from these popular and democratic elections will therefore be that the Plt is less likely to pass oppressive legislation because of acct etc.
I authored the Jacaranda online text, StudyON. I don't want to undertake anything that huge again in the near future, but I do seriously want to do some updates to it! This year I'm mainly just doing the CPAP lectures, updates, sacs, etc etc!
BTW I'm on my phone so proofing is too much trouble - I'll just apologise here in advance in case! Good luck tomorrow