Victoria's legal system is a very broad topic. You might find it helpful to narrow in on one or two areas of interest. Have you decided what those might be yet? 
Seconding Remi's idea, terrorism has been a bit of a hot topic lately. You might find the following articles of interest:
Victoria to crack down on potential terrorists after Brighton siege
Ministers to be hauled before Supreme Court after criticising terror sentencing
Try and find a particular area of your topic that you would like to explore. This will help to narrow things down, and you'll find that gathering statistics, facts, etc. will be much easier, and so will getting started.
As Coffee said, focusing on "Victoria's Legal System" as a whole will be difficult. I had friends who chose this topic and struggled because you simply cannot address EVERYTHING in 5 minutes (assuming you have a time limit) and will struggle to set up a good argument. You'd be better off choosing a smaller aspect of Victoria's legal system and working on arguing for/against that.
What I learnt was that you should pick a relatively simple issue that you can argue clearly and concisely. By choosing a topic that doesn't require much background knowledge, you can use your time effectively to set up a really good, in-depth argument and won't have to spend time explaining your issue, which could make your oral seem more informative than persuasive. ( I learnt this the hard way

)
Finally, delivery is key! My oral itself wasn't fantastically written, but I practiced my delivery about 10 times, timed and all in front of family and I think that boosted my mark more than anything. Even if you don't feel confident about your oral, you need to sell it to your assessors and you can do this by having a solid delivery. Vary your tone and gestures throughout (and mention this in your SOI!) and you can really sell your oral as persuasive.
Hope this helped!