I found this to be the most useful guide ever https://www.reddit.com/r/ABraThatFit..._as_it/c469bi0

I followed the directions to the letter, and had much greater success buying bras online afterwards.

I'm guessing Perth, is Perth, Western Australia...so this chart might be handy for you:
http://www.brasnthings.com/sizing-gu...ize-conversion