I think it's that Aikikai Australia can conduct
dan gradings and award dan ranks on behalf of Hombu. I've only ever trained with Aikikai, I've never even met someone who practiced a different style in real life, so this might be splitting hairs, and may very well be incorrect.
It's also the only Australian organisation listed on the Aikikai Foundation website, if you believe things published on the internet: http://www.aikikai.or.jp/eng/info2/overseas.htm
I'll ask someone who would know more than me