Is there a super close substitute for graham crackers?
First off, try digestive biscuits. They're really very similar cookies/crackers/biscuits, and it's a common enough substitution that it's even mentioned on Wikipedia. Cook's Thesaurus implies that they're called wheatmeal biscuits in Australia.

sourd'oh's suggestion of particularly crunchy gingersnaps might work too, but you'll also want to make sure they're not too spicy!

(Unfortunately, I don't know about specific varieties since I'm not Australian.)