I think the total if declared over $1000 AUD is 14%. 10% GST plus 4% duty fee.
Last time I had an excisable item sent tome, they sent a letter in the post (Customs and Border Protection) telling me the amount. I then went down to Brisbane (Customs is near DFO at the airport) and paid it at the counter, they gave me a receipt, then I went and collected it off the back dock. No issues.
If someone sends it to you declared as a second hand mount, value under $1000,expect a request for original invoice/stat dec from the seller. If they say it's a gift, well....
Customs do check values online
