International mail to Australia gets seized much more regularly than mail to other countries. No package is immune from being seized. I have heard of people in USA who had letters without drugs in them opened and inspected by customs, so in all cases there is a probability that your package will be randomly inspected. The best you can possibly hope to do is try to keep the chance of thorough inspection equal to random chance selection. Australia thoroughly inspects more international mail than most countries do, and according to all the documentation available they do two stage screening on 100% of international mail.