I've been hearing about a lot of problems with the Australian CE for Factions. If you get the PreOrder key, that would be the only way you can get the Head Start, so then it wouldn't matter if you didn't have the Retail Key.
I'd also recommend checking out the Dev Tracker section of this site, as there's a lot of information there on this problem.
It will be downloaded just like all other updates. So, all you need to do is put that preorder key onto your account, and you will have access to the Head Start Event.
If you can get the preorder items with your characters, then it's all set up and nothing more needs to be done. Just remember to add the Retail key when you get that, as the preorder key will only give you access to Cantha through the 28th.