In theory the law is gender-neutral, and the legal situation is covered briefly in this article:
In practice if you present as female you're going to get more hassle when topless than if you present as male. But it's likely to be hassle by random bystanders rather than the cops unless you break rule one: don't be a dick. Shirt off in Newtown basically any time... no problem. Shirt off in St Andrew's Cathedral... even Tony Abbott wouldn't go that far.
Technically it's an expectation of "public decency" which means you really are caught between an increasingly laissez-faire general public and people who are upset in general. The most likely problem is a Karen ringing the cops and the cop saying "please make my life easier by covering up".
The world naked bike ride and other similar events generally allow/encourage toplessness in women, but legally that's a "nature of the event" rather than "anyone can go topless anywhere".