That Australia and their likes could survive nuclear winter is a subject of probability rather than fact. Could this be the reason houses are more expensive in Australia?
In the Universe, we call it a great filter because the likelihood is that no advanced civilisation managed to develop beyond that point for whatever mysterious reason (war, disease, etc which becomes inevitably destructive). Are we approaching that filter?
Are you talking about the carrying capacity of the Earth or de-civilization?
Could be anything. The idea is that there's just some universal barrier preventing civilisations from ever getting to a point that we become interplanetary.
If your theory is right, perhaps the great filter will annihilate all but Australians, who will then go on to singlehandedly colonise the galaxy, bringing with them death and destruction in the form of various deadly spiders and insects