I only just downloaded Itunes myself so I am not sure about all of the ins and outs but this is what I found. I think you have to create a different itunes account from the one you already have. I am going to try it tomorrow and see if it works.
"Q: Can I purchase an iTunes card in the UK for use in Australia?
- Anonymous
A: iTunes Gift Cards can only be used for the iTunes Store for the country in which the card was purchased. This means that an iTunes Gift Card purchased in the U.K. can only be redeemed in the U.K. iTunes Store. That said, Apple does not limit access to the iTunes Store based on a user’s physical location, so you would be able to access the U.K. iTunes Store from Australia, or anywhere else in the world for that matter. However, this is probably not something you want to do on a regular basis.
Firstly, from a strictly legal point of view, Apple’s Terms and Conditions limit you to only using the iTunes Store in the country you’re in. For example, the iTunes Store U.K. Terms and Conditions note that “The Service is available to you only in the United Kingdom. You agree not to use or attempt to use the Service from outside this location. iTunes may use technologies to verify your compliance.” Similar language is used in most other countries where the iTunes Store is available. If Apple decides that you have been using your iTunes Store account outside of the Terms and Conditions, it has the right to shut down your account, possibly resulting in a loss of access to, or updates for, any items that were purchased with that account.
In practical terms, since an iTunes Store account is created for, and generally tied to an existing country based on the billing address and/or gift card used, by using a U.K. Gift Card in Australia you would effectively be setting up an account for the U.K. iTunes Store, requiring all future payment methods to be U.K. based—either a credit or debit card issued by a U.K. bank or additional gift cards purchased from the U.K. While it is possible to use more than one iTunes Store account on a single computer, this is generally more trouble than it’s worth compared to simply using an Australian iTunes Store account in the first place, particularly if you’re purchasing iOS applications as you will need to switch back and forth between the accounts to receive available updates. Note that it is also possible to change the country on an existing iTunes Store account, however this requires supplying new billing information for the new country and may also prevent you from re-downloading content or updating apps previously purchased from the iTunes Store in the original country."
http://www.eyeondemand.com/2013/02/26/how-to-creating-an-itunes-account-for-a-different-country/