"A large selection from popular series by Michael Connelly, J. D. Robb, Diana Gabaldon, and more! Sale ends October 17, 2016 at 11:59 PM PT (US)."
I'm not sure if the AUS marketplace also has it, but this one is for the US one. If you have an AUS account, you can still take advantage of the offer but to access the audiobook, you will have to log out of your account in the Audible app and log in again but at the US marketplace instead of AUS.
I get told the offer is not available. My account is a US based one but still doesn't work.