Goods imported into Australia are subject to Customs Charges: Duty and GST. However, all goods (except for tobacco products and alcoholic beverages) may be imported duty and tax free if their value is $1,000 AUD or less.

What this means is that if Customs determines that the value of the goods when they enter Australia is less than $1,000 AUD ($~861.10 USD) then they don't charge you anything.

One thing to be mindful of is that Customs takes the USD values supplied by the sender and uses those to work out the AUD value at the time the goods are imported. Between the time you order and pay for the goods and when they arrive in Australia the currency might weaken against the USD which could push you over the $1,000 AUD limit. To guard against that happening it would be advisable to allow for a 10% drop in the value of the AUD, i.e., limit yourself to spending say $900 AUD ($~774.99 USD).

For the full details please visit the Australian Customs Service web site.