Well I did natpol this year, and yes it is pretty much only based on Australia with only a little of the US and if you choose to write on foreign affairs, knowledge of regional events.
AOS 1 - Comparing AUS and US in regards to executive, judiciary, legislative workings as well as how democratic they are.
AOS 2 - Looking at change in the australian political institutions
AOS 3 - Formulating Australian public policy
AOS 4 - Regional foreign policy
Only contemporary knowledge is required for all of the areas of study. I found that very little additional research was needed outside of reading the newspaper and using the textbook.
Essentially for the exam, you really only need to specialise in two areas, so if you weren't that confident, you could always focus on AOS 2+3 for the essays (exam is made up of short answer on each aos and 2 essays - Unit 3 & Unit 4), so you wouldn't have to know much about other countries at all.
So yeah, if you have little knowledge of foreign affairs or just aren't as fond of it as australian affairs, go for natpol.