As mentioned in this reddit thread:
A slickdeals bargain allows you to get 12 credits for one year. Much cheaper than the monthly plan.
This offer is for new members only but a number of current members (inlcuding me) were able to go to Audible's live chat and get this plan too. I'm on the US membership. Please comment below if you're able to do the same with an Aus. membership.
I gave up my Platinum US account last year and now buy credits from here https://exploreaudiobooks.com/ and have never had an issue with them.