Hi Tandum,
As far as I know telescopes are duty-free. See
CHAPTER 90 and
CUSTOMS TARIFF of Customs Tariff Act 1995. "Telescopes" are listed under "9005" ref #.
So only GST is required if the total price is over $1000.
Are you sure that the price from Astro Optical with GST included?
I've got slightly cheaper price from another company but it says "plus GST".
cheers,
Mike