From what I remember on the website it's 10% of the total worth in AUD of the item above $50AUD. http://www.customs.gov.au/site/page.cfm?u=4376

They won't necessarily charge you. It's up to them to determine whether it's a genuine gift and waive the charges. It's more common for this to happen on regular post items. If you get something sent via evil FedEX, UPS, or DHL they'll definately hit you up with tarriffs.