@Happyladydiver: I don't know what the rental BCDs are like in Australia, but in the U.S. they tend to rent the ScubaPro Hydros or a lower-end brand, and I've never had a rental that was a women's BCD. Something that worked for me was to simulate what I would be wearing under the BCD. I walked in wearing a thin material-type shirt to simulate a dive skin and brought my 3 mil wetsuit jacket, then tried on the BCD. The shop owner spent a lot of time getting me fitted and adjusting straps. The thing with dive equipment is that you can't return it once it gets wet except for defects, so your best option is to get the right fit in the shop.
There's a section on SB called Women's Perspectives, and you might find useful info there, too. Great group of gals on that forum.