Australia and the US have a free trade agreement therefore you will only pay GST(10%) and some minor process charges from broker. No Stamp Duty. UPS is a great option as they have there own tracking system. They will contact you when guitar arrives, get credit card details and deliver very quickly. They only have a small warehouse and want to get stuff out quick. eg $2000 guitar probably cost around $200 - $300.
We have a free trade agreement with several countries, so check before you buy.
This information was given to me by Australian Customs 29/06/11
HOPE IT WAS HELPFUL