From the WIA, original post here.

Date : 19 / 02 / 2016 
Author : Fred Swainston – VK3DAC

The ACMA will cancel your callsign if it doesn’t receive a response to your renewal invoice within a limited time after the expiry date.  The ACMA will lapse the licence, and your callsign will then be available for re-issue.  A few people try to get their licence back after it has expired, but are told by ACMA to apply for a callsign through the WIA Office to re-start the process.  This means that the WIA needs to make a callsign recommendation and submit this to ACMA with a licence application, to enable a new licence to be issued.  However this is not always under the previous callsign, because as we have explained, it may have already been issued to someone else.

The answer is to always know your ACMA Amateur Licence renewal date, whether or not you receive formal renewal, and make sure you renew it to keep the callsign.  ACMA’s Amateur licencing process relies on you keeping your contact details current with their office.