Customs is a government agency responsible for regulating shipments entering a country. All shipments being sent to and from a country must clear customs first. Merchandise exported into another country must clear customs and may incur a customs duty.

Note:
As the seller, you may need to pay additional fees if items have to be returned because they don't clear customs. Also, goods can be held up if your carrier does not arrange for a customs broker in advance. (Most carriers provide this service automatically).

Customs documentation

Accurate documentation such as an invoice is required for shipping anything other than letters and documents. Your shipping carrier will be able assist you with appropriate documentation.

The Australian government has a number of resources to promote international trade and to help businesses of all sizes familiarise themselves with export regulations, and shipping companies offer special services to businesses that regularly ship goods overseas. Information about these resources can be found at:
Australian Department of Foreign Affairs and Trade
Austrade Online
Australian Customs Service