Combination of the deals here and here
10% off macs at dick smith, plus 10% extra value on gift cards at coles = 18.2% off!
The coles extra 10% credit on Dick Smith gift cards was extended (thanks Mr Rort)
Page 12 of the Hearldsun (3 sep 2015) coles advert states that the Dick Smith 10% bonus is extended until 06/09/2015 whilst stock lasts.
It's actually 18.18% off.
Take a basis price of $100.
10% off is $90.
To get a giftcard value of $90, you have to pay $81.81
110% x $81.81 = $90
QED.