Rob,
I have bought heaps from overseas.....I have used 2 different retailers so far...these are:
http://www.optcorp.com/
http://www.skiesunlimited.net/index.php
Both retailers have been excellant to me but I do find Bob from Skies unlimited to be alot quicker regarding a response to prices and purchases and generaly as a contact by email.....BTW Bob is Astro_Guy on this forum.
Regarding Tax.....any puchase over $500AUD includding postage costs brings you tax payable of the GST...so 10% tax will have to be paid before the parcel is delivered.
Regarding Custom's......I will try to keep it simple.....if you use the goverment run postage system eg.....USPS = United States Postal service you are allowed a total of $1000 AUD includding postage costs to enter Australia without paying the $50 customs fees.....
If you use a Privately run postage couriers eg..UPS meaning United Postal Service the $1000 limit is reduced down to $250.
I use USPS if my parcel goes over the $250AUD
http://www.usps.com/
I use UPS if my parcel is under $250AUD
http://www.ups.com/
Just remember anything over the $500 does attract the GST no matter who you use and the $50 Customs fee varies depending who you use.
Hope this helps.....