Many books are cheaper than their UK sister site, particularly their pre-orders:
Dust of Dreams £9.16 vs £15.00
The Girl Who Kicked the Hornet's Nest £8.70 vs £14.24
Dark Slayer £5.95 vs £9.74
(prices listed are pre-discount and subject to change)
Exact same code has already been posted before:
http://www.ozbargain.com.au/node/14172
In addition, the expiry date of July 31st 2009 was mentioned.