Courtesy of Link, who managed to find out MightyApe's Armageddon Auckland code for this year. They've been much more secretive this time around!
Also doesn't seem to work for all purchases - eg. I tried applying it to the Sega Mega Drive Mini console and it didn't work, but it did work on an Xbox game.
Update: added expiry date, 15 Nov, from competition email that I received today:
As another Thank You for visiting us and entering our competition, we have an exclusive promo code just for you. Use the promo code DOOMSLAYER to get $10 OFF your order from now until Tuesday 15th November!