3 month trial for new members of the Australian Audible site, where you get one book credit a month, and are yours to keep even after cancelling the subscription. Subscription will renew automatically after the 3 months at $16.45 a month.
T&C
This promotion is open until 30 November 2018 23:59 AEDT, for new customers of Audible in Australia or New Zealand who sign up for an Audible trial membership at www.audible.com.au. You'll get one title per month for $0.00/month, for the first 3 months. After the first 3 months, your membership will renew automatically at the regular monthly price, currently $16.45. Cancel anytime. The promotion applies only to items sold by audible.com.au. The same items sold by other sellers (e.g. audible.com, or audible.co.uk) do not qualify. Claim codes cannot be resold, transferred for value, redeemed for cash or applied to any other Audible account, except where required by law. This promotion may not be combined with other offers. Limit one per customer and account. Audible account required. Other terms and conditions apply, including those available at http://www.audible.com.au/legal-terms/Conditions-of-Use?modu… and claim code terms and “conditions at https://audible-au.custhelp.com/app/answers/detail/a_id/7015…. If you violate any of the terms or conditions, the promotion will be invalid, and the offer will not apply. Audible reserves the right to modify or cancel at any time. If your credit card is declined, the offer will not apply. Any terms prohibited by law shall be void without prejudice to the remaining terms.
I don't think it is exclusively for Amazon Prime members, happy to be corrected though and will update the deal accordingly.