I got two ways one is check the games area not demos and if that does not work read the rest of my post.
Sign up to the US version of playstation.com now log into your new account using the login stuff(from us playstation) same way like your australia psn.
go to the store(in games not demos).
download and install(ps3 games are region free).
Note: after this make sure you sign back into australia psn because US firmware can crash a ps3 from a different region.