As per this site.
http://www.customs.gov.au/site/page.cfm?u=4376
Duty is applicable on items over 1000$ as per Customs Tarrif Act.
As per Customs Tarif Act (do search on page linked for Telescopes).
http://www.austlii.edu.au/au/legis/c...8285/sch1.html
Astronomical Instruments (non radio astronomy) are free of Duty.
Above is also available here
http://www.customs.gov.au/SITE/page.cfm?u=4273
Under "Chapter 90 Optical, photographic, cinematographic, measuring, checking, precision, medical or surgical instruments and apparatus; parts and accessories thereof"
Sub section "Goods (9001 to 9021.10.20)"
see linked pdf
http://www.customs.gov.au/webdata/re...ff/ht90aw1.pdf
Loof in pdf page 3 for 9005 this is the section pertaining to Astronomical Instruments.
Hence the Calculation should only be
Price + Shiping&insurance + 10% GST.
Regards