Australia has strict quarantine requirements regarding importing animal and plant derived products (any food, wooden products, seeds, etc.) You must declare all such material, even if the items are permitted, and baggage is frequently scanned and may be examined by dogs. You may be fined $220 on-the-spot if you accidentally fail to declare, or even prosecuted in serious cases. Declared material will be examined and, depending on the circumstances, may be retained, disposed of, returned to you, or treated by quarantine at your expense. (You may have to pick the item up at a later time.) Processed and sealed chocolates and other confectionery are usually permitted after being declared and examined, as are reasonable quantities of infant formula with an accompanying infant. Different rules apply depending on the origin country of foods, as well as the state in which you are entering Australia.
Some shells, coral and items made from a protected species are also prohibited to discourage the trade in items that may originate from a threatened ecosystem or species.
While there are no restrictions on the amount of money that can be brought in or out, Australian customs also requires you to declare if you are bringing $10,000 (Australian) or more (or its equivalent in foreign currency) in or out of the country and you will be asked to complete some paperwork. Not declaring may result in you being arrested and a possible seizure of the cash.
(Thanks to WikiVoyage.org)