In my best Tony Montana voice...

On eBay, first you get the business policies, then you get the regional block, then you get the money (sometimes, hopefully).

But yes. In all seriousness, you do need to have business policies activated on your account to access the option to block regions of Australia. Not everyone is able to opt in, but you can try via this link: 

http://www.bizpolicy.ebay.com.au/businesspolicy/policyoptin

You might also want to check the ebay help pages etc for exactly what business policies does, and how it works, because a lot of people don't like them. (They basically mean you create global settings for postage costs, returns policies and payment methods, you can have different policy profiles, assign them to different listings, then edit the policies in My eBay rather than by individual, or even bulk, listing(s)).