When I was there, a year ago, it was about 2 Reais to 1 Aussie dollar.
Rio is relatively expensive compared to most places in Brasil.
If you are living in a hostel you can find dorms for R$20-25 a day, you can pay about double that for single rooms. Food is rather expensive in Rio but there are plenty of supermarkets and little hidden food places.
If you are on a budget stay away from the general tourist eateries.
You can get away with about R$500 or so a week living comfortably (that is about A$300 current exchange rate) almost 8 years ago