Thank you everyone,
Trippin - I am on VIC, and here at least the surcharge, across the majors at least, seems to be about $30/ day. Although I notice in NSW there seems to be no surcharge, which is odd.
Cynicor - I took your suggestion and booked using an AWD (Avis Worldwide Discount) code, which changed the age requirement to 21 and thus avoid the surcharge, plus a discount on the rate. Great result!