Port Arthur. The theory goes that Martin Bryan was actually a patsy placed by the CIA to allow a government kneejerk reaction to disarm the citizens, making Australia a more compliant ally, and easier to invade if need be.
After having lived in Tassie during that time, I really doubt that one. Have heard other conspiracy theories about the massacre, but that is the first time I have heard of that exact one.

Don't have a favourite theory at the moment. My Mum used to love them.