Just like your drivers license or passport, domain names need to be renewed. The renewal purely covers the domain name only and does not cover other services that may be required to actually make it work (such as DNS Hosting).
.com.au domains are renewed every two years and can only be renewed for two years at a time. This is the period set by the AuDA, the body that governs domain names in Australia.
.com, .net, .co and other domains (that do not contain .au) are normally renewed with us every two years, however they can renewed for any period between 1 and 10 years.