I called the Microsoft Promotions line this morning to get some clarification on some details of the offer.
Each individual product item requires one claim form to be filled in, so you can only claim on 3 items total. eg. If you buy 2 controllers, a hard drive and a wireless adapter in one transaction, you can only claim on 3 of the items. It is not one claim per receipt. You need to decide which items you wish to claim, so use your 3 claims wisely.
You will get the full Cash Back for the items regardless of what you pay the retailer, provided the discount the retailer already offers isn't an official Microsoft offer. You don't have to pay RRP to get the full Cash Back, so find the cheapest Australian based retailer to purchase your XBox 360 accessories. Online stores are OK, but just make sure they are Australian retailers. If you want to play it safe try the retailers listed at
http://www.xbox.com/en-AU/live/wheretobuy/.
The online redemption form currently is not secured (as mentioned above), but since it has only gone live in the past few days I was told this may be rectified in coming days. They will not take your details over the phone. If you are uncomfortable with giving your bank details over the web on an unsecured link I was told you can be sent a cheque instead by entering all 0's (zeroes) in the Account Number and BSB fields. In the Account Name field put the name you would like the cheque made out to, and in the Bank Name field you can enter the word "cheque" and they will send you a cheque.
Call the Microsoft Promotions line 1800 233 033 if you need further help.