All goods imported into Australia are subject to tax. However goods of low value are generally released by Customs for direct delivery, this is known as the Customs screen free limit.

Customs Screen Free Limits

Goods	Sales Tax Rate*	AU Dollar Limit	US Dollar SAFE Limit*
Audio CDs and Video DVDs	22%	$189.39	$110

Thus to be absolutely safe and ensure you don't have to pay any sales tax you should keep the value of the goods purchased below US$110.

Information about Books is coming soon

* Information collated from Shopping on the Internet: Facts for Consumers published by the Australian Government. Limits based on the minimum $50 charge rate on stated sales tax plus a 20% uplift. US SAFE Limit based on a low exchange rate of 0.60