My understanding is that you're correct, however, check out the Thai embassy page (Canberra).. at the very end of the page.."Please note that visa exemption can be used 2 times per calendar year."
From the Australia Thai embassy website "If you do not hold a Thai driver's licence and wish to drive in Thailand, you need an international driving permit (IDP), as well as you're Australian licence. If you drive without the correct license, you could be arrested or jailed."