You cannot use Australian bills as candles on a birthday cake. They are made of plastic and will not burn. However, they do come in an assortment of bright colours such as red, orange, green and white with really cool clear plastic windows and a hidden silhouette you can see when you look at a bill with the light behind it.
I remember when they first introduced the plastic bills. The Prime Minister autographed a bill in great pride for someone at the launch and promptly got himself into trouble. It is illegal here to deface an image of the queen and of course he had just signed over her face.
Another fun fact is that the two dollar coins are not solid metal, but filled with plastic and minted in Korea. Try obtaining one and two dollar Australian coins and then dropping them. The one dollar coin will make the expected "ting" sound, however the two dollar coin will just go "clack".