I'm an intensely proud Aussie, but I can understand how flags hanging off the balconies makes your Building look just as unkempt as it does when residents hang laundry items such as sheets and towels.
So why not take the approach that the same By-Law that applies to laundry items applies to flags.
That would mean that you are not stifling national pride by prohibiting your patriotic residents from flying the national flag for their own gratification (why else?), you're just asking them to position those flag/s, including on Common Property, such that they're not visible from the outside of the Building.
Why not offer to put a Agenda Item before the next General Meeting of your Plan to put a suitably sized and properly illuminated flagpole on your Building, and proudly fly the national flag there?