As per the auDA policies, the .au direct domains can only be Registered and Renewed for 1,2,3,4 or 5 years.
Documentation reference links :
https://www.auda.org.au/policy/domain-renewal-expiry-and-deletion-policy-2010-01
https://www.auda.org.au/au-domain-names/domain-name-help/renewing-au-domain-name-licence-fact-sheet
If you have any problems or need more help, please open a support ticket so our staff can assist you!