Works on Australian audible.com accounts despite the terms and conditions stating only for is only available to customers in the United States. The credit expires on the 21st of March 2015.
To get the Whispersync discount, you may be able to use the credit in conjunction with Amazon's $5 eBook Credit for registering the Kindle for iOS or Kindle for Android app for the first time. Note: I haven't heard of anyone getting the credit automatically but after contacting Amazon, they should manually apply it.
You can also just borrow the kindle version from a US Overdrive library to get the reduced price.
Also if your Amazon account is set to the US for digital content, you can use Family Library to share Kindle books, apps, and audiobooks. Choose wisely, the adult account you add will have access to your credit card and you won't be able add another person for 180 days if they remove you.
Remember if you grabbed the previous $10 credit deal, it expires on the 25th - https://www.ozbargain.com.au/node/171731