I've been looking in to the idea of a pre-paid cash card. You load your currency on it at home and then can use it like a debit card in shops, restaurants and to withdraw cash. Travelex does an Australian dollar one.
As to how much to take - it's a tricky one as it depends how much you eat / drink, how many souvenirs you like to buy and how many excursions you want to do. The brochure can help you with what meals are included each day so you can work out what you have to buy yourself. If you look at some of the other Australia posts there is a guide as to what other excursions are available and their cost. For my trip I intend to take half cash, half loaded on a prepaid card, along with a credit card for emergencies.
Hope this helps
Sally