The Australian Society of Authors (ASA) also includes the Society of Book Illustrators (SOBI) so represents both authors and illustrators.
The Australian Cartoonists Association (ACA), predominantly represents editorial cartoonists, comic /cartoon strip artists and comic book illustrators. The Stanley Awards has a Best Comic Book Artist Award, which Jozef Szekeres, Tim McEwen and I consulted on several years ago to define the guidelines.
The ASA and the ACA has a reciprocal agreement so that members from each can get benefits from the other organisation’s activities and projects at special prices. Several comic book artists are members of both.
I don’t know much about Illustrators Australia except they used to put out a great book every year to showcase their members’ works – it’s probably an online portfolio now.