You last post is incorrect.
The duty/GST limit is $900, but:
Let's say you buy a camera $700 in Australia, plus(separately), a lens which is $400. The total exceeds $900(total=$1100, exceeding the limit by $200), so you would have to pay the FULL duty/GST on the item that falls above the limit.
So, you DO NOT pay for the camera ($700) but you do pay for the lens, not on the amount above $900 (which is $200), but on the whole $400 amount.
I'm not sure, but I think if you purchased both on the same receipt($700+$400) as a single pruchase, a pedantic customs officer MAY view that as one purchase, and charge duty/GST on the whole $1100, but I couldn't be sure.
Anyway, in addition, let's say you buy something in Japan for $200.
The total is $700 + $400 + $200. You can group the $700 + $200 together (total =$900), and still just pay the duty/GST on $400.
For alcohol, if you exceed the allowance, you pay duty/GST on ALL of the alcohol. Same applies to cigarettes.
Realistically, if you declare what you have, they will often be lenient.