There is no export duty. The Australian government will be happy you're spending your money in Australia, and as mentioned, you can even get the VAT tax back, if you care to learn the process and fill the right forms.
Duties are due when entering a country, not when leaving. When you arrive at your destination (where the bought tablets will stay), you might ...
Rule of thumb, declarations are made to customs, after passport control.
If you're changing airports in Istanbul, ie from IST/LTFM to SAW/LTFJ, you'll be going though customs. Then you will need to declare.
If you're remaining in the same airport transit area, you won't need to declare.