It's the wearing of the basest form of casual dress at all times and the derision of anyone that makes an effort to be neat and tidy. If I have a shower and put on an inexpensive button up shirt and a pair of cleans jeans and go down to the pub, people ask me why I am dressed up. Apparently, how dirty and filthy you are is a mark of how hard you work, but in my fathers era - the forties and fifties - men in rural areas worked harder than they do now, but if they went to town or went out, they would clean themselves up and wear good clothes. The wearing of thongs (flip flops), wife beaters, baseball caps and flannos is considered the true Australian way to dress, but now with all these white guys wearing tribal tattoos on the top of their arms, and with the baseball cap thing, they don't look that different to Rap Gangsters. It is basically dressing like a slob. I'm a bogan myself, but trying to reform myself The reason that bogans don't dress that well is basically because they prefer to spend their money on grog and cars.
It depends on where in Aus you are and what class you belong to. I certainly don't know anyone who wears flip flops or a flannel check shirt.