If FSD after purchase is AUD$11400 and $8500 if included in the purchase, then if the car is already in the LCT bracket (above $75526) then by my calcs, adding at purchase would put the FSD at $11605 including LCT), so only $205 extra to include FSD at purchase time. Assuming that it is included in the LCT taxable amount, which seems to hold up when looking at the Model S financing numbers on the AU website.
Perhaps a model S or X owner can confirm that.