Stamp Duty is an insidious tax....
Just last week I spoke to the ATO regarding being charged GST on the premium after state stamp duty was added on my BUPA pet insurance - wouldn't normally bother with the insurance but our dog likes eating blowfish! House insurances etc charge GST on the premium before stamp duty but apparently there was a ruling a few years ago that allows them to calculate the GST amount on the total amount.
The reason went through to the keeper because I'm no accountant - 2 friends who are accountants and the accountant at my work all reckoned that GST is calculated pre addition of stamp duty - but suffice to say that the Feds would be very happy with the extra sneaky 3 or 4% on top of the legislated 10% !