What happens when my .com.au domain expires?

When a .com.au domain passes its expiry date unpaid, the damage arrives in stages:

• Immediately — the domain stops resolving: the website goes down and email to the domain starts bouncing
• The grace window — under auDA’s licensing rules an expired .au name is purged about 30 calendar days after expiry; until then your registrar can still renew it for you, sometimes with a late fee
• After the purge — the name is released and anyone can register it, including competitors and squatters who watch expiry lists for exactly this

Renewal failures are almost always boring: an expired credit card, or renewal notices going to an inbox nobody reads. Turn on auto-renew, keep the payment method current, and make sure the registrar account email is one a human checks.