It's not an ultimate deterrence to invasion. MAD is a theory, not an iron law of the function of the universe. If there's one thing World War II taught us, it's quite possible for modern life to continue despite 8 figure casualties, and quite possible for industrial output to increase even under the most severe conditions.
Australia is hard to invade because it's a separate continent, not solely because it would be protected by nuclear weapons. It's all the other normal stuff related to the challenge of conducting opposed landings over long distances that would make it very costly to attack. The nukes are a plus but they are not magical. All modern nations have tons of expendable population. It's politically convenient to present nukes as the ultimate deterrent but it's not backed by empirical evidence. The other issue is that due to test ban treaties we really do not know how currently existing stockpiles of such weapons will perform under real conditions. We could have vast stockpiles of antiquated garbage. We just don't know.
Australia is hard to invade because it's a separate continent, not solely because it would be protected by nuclear weapons. It's all the other normal stuff related to the challenge of conducting opposed landings over long distances that would make it very costly to attack. The nukes are a plus but they are not magical. All modern nations have tons of expendable population. It's politically convenient to present nukes as the ultimate deterrent but it's not backed by empirical evidence. The other issue is that due to test ban treaties we really do not know how currently existing stockpiles of such weapons will perform under real conditions. We could have vast stockpiles of antiquated garbage. We just don't know.