I'm Going Backpacking For A Month In Australia, What Do I Need To Pack Into My Backpack?

If you were planning on backpacking, one of the first things that I would ensure, is that you have a great backpack, something with comfortable straps and many pockets and something that breathes, so that you don't have an incredibly sweaty back all of the time.

Regarding clothes, you will want light clothing, t-shirts and shorts mainly, but with a pair of jeans and one light jumper, a thin coat which is waterproof might be useful, swimming costume of some type might also be good, a nice shirt and trousers/blouse and trousers/skirt for a nice evening and a pair of good walking shoes/boots.  You will want light cotton undies and thick socks and possibly a thong/flip flop/teva style shoe.

You will need towels, wash bag with toiletries etc.

You might also want a water bottle, salt tablets for potential dehydration, sun cream, after sun, a small first aid kit with plasters.  You might want a swiss army style knife, definitely sun glasses.  

Since you might want to travel through hostels, take a sheet, a pillow case and some bug cream.

In the U.S., backpacking usually means travelling outdoors in the national or state parks, carrying everything you need on your back. That will include shelter, food, clothing, hiking and emergency gears. You (and your companion) will have to be totally self-sufficient, and not count on help from any other source. But that is the allure: The complete freedom and solitude.
Outside of the U.S., including Australia, backpacking means travelling from place to place, usually staying in hostels, campgrounds, homes, etc. They also carry most everything they need, but do rely on public transportation, local stores for supplies, and other packpackers they meet along the way. There is freedom of movement and social interaction. The adventure is the allure and reward.
The demand of each type will determine what you pack in your bag. Weight is a big factor, but more so for the mountain traveler, who will opt for the lightest, most durable and multi-function gears.
Just an aside: Bluejeans are not favored by either group nowadays. They are bulky, heavy and a huge burden on you when they get rained on. Similarly for cotton clothes. Most people prefer the modern fabrics.
There are dozens and dozens of good Websites for detail information. For backpacking in the wild, try www.rei.com, www.golite,com. For hostelling in Australia, try www.yha.com.au and many others.
Good luck and have fun.