Importing items into Australia (Buying low-value, non-prohibited items over the Internet and shipping to Australia) is straight forward.  If buying from the USA, do be sure the seller packs and ship with proper customs declaration form CN22 for values of $400US, and that the item is not restricted by Australia for import.  It is the buyers responsibility to determine that the item you are buying is not restricted by Australia.

AU buyers buying over the internet:

    * If you are receiving your goods via the post you are considered the importer of those goods and these may be subject to Customs duty and Goods and Services Tax (GST).

    * Customs advises people purchasing goods over the Internet to be aware that, when those goods arrive in Australia, they will be subject to Customs controls.

    * Certain goods brought into Australia require an import permit. Customs might seize or detain such goods pending presentation of the permit. Alternatively, the goods might be a prohibited import and not allowed into the country under any circumstance.

And the good news for non-restricted imported goods with a value of A$1000 or less

    * Generally, all goods imported into Australia are subject to Customs duty and GST and are assessed for community protection risks. However, all goods (except for tobacco and alcoholic beverages) may be imported duty and tax free if the value of the goods is A$1000 or less.

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Source:
(you must cut-and-paste since eBay does not allow links in their guides>)

http://www.customs.gov.au/site/page.cfm?u=5549