Hi Sharon, and welcome to the site. I've been away so it's a bit of a belated welcome .. but still sincere! :)
Yes, it's correct that an Australian postcode is required to register for the site. It helps us minimise the ongoing spam (which mostly originates internationally) which all online networks are subject to. However, we don't really want to block genuine users so we're looking into what's available to open up access a bit more. In the meanwhile, if you have any friends or family who'd like to join who don't have an Aussie postcode, just let me know and we'll work something out (they will be able to see public posts without registering, but will need to register to comment).
Nice to see you here -- just shout if you need a hand with anything.
Di