Really depends on where you are going as expense can vary depending on where exactly you are planning on staying? Also are you planning on staying in one place or are you planning to travel?

If you are planning on staying in one city you will not have the incurred expense of travel by which whatever method of transport taken tends to be expensive.

For hostel pricing you are looking at $22-30 AD per night.

Hope this helps a little.

My sister and I did 3 weeks in Sydney, Cairns/Port Douglas, and Canberra really cheap. We had the advantage of having our Uncle who lives in Canberra, but Cairns and Port Douglas we did a week with 3 day trips (Daintree Rainforest, White Water Rafting, and Great Barrier Reef) for about $500 USD. That included everything from flights, food, hostel, excursions, and anything else. You can do it on a budget for sure. Just make sure that you do your homework and figure out prices in advance and have a plan. When my sister and I went, we both had very limited money because we both graduate from college about a month before our visit. Good luck and enjoy!