Not much. Typically there is a 60 day vesting period (I am not sure if that is the correct length or name) when the domain is still in the control of the registrar and the owner can pay them a fee (it was $100 for my previous registrar) to get the domain back.
I've been involved in trying to rectify this sort of mistake before...
At least with .com.au domains, there's not a lot of risk of losing a recently expired domain, what you do suffer is a ~$100 vig for the registrar to "expedite the re-registration"...
Some companies will squeeze money out of everything when they get the chance, this reminds me of a €13 'administrative processing' surcharge I recently got on a €7.50 invoice I forgot to pay.
http://www.icann.org/en/registrars/gtld-lifecycle.htm