I got fitted at my local "larger lingerie" store and was a 20GG when breastfeeding. I can't remember exactly what brand they were as I've now disposed of them, but they WEREN'T CHEAP! The store I used was "La Figure Lingerie" but I don't know whether they are in Sydney or not.

Best wishes, the right bra is out there, you just need the right advice to find it. I'd definitely avoid underwire though.