You cannot wipe out the planet with nuclear weapons, there just aren't enough. You can do significant damage if you send enough in one area, but consider how large the surface of the planet is and how many nukes we have now, it wouldn't work out. Take the cold war scenario where if the soviets and nato were to nuke each other. All the beligerent countries involved would collapse as well as any country they supported. With all the UN security council members nuked, the dictators of the world are free to go crazy on each other. There would be a huge impact on the global economy with mass famine virtually everywhere. And all those volatile areas of the world like pakistan and the mid east would fall into anarchy. You see, the threat isn't so much the damage of the weapons which would kill over a half a billion at least if this occured, but the resulting vacuum of power that would leave much of the developing world in chaos. Why would anyone target a nuke at new zealand, the weapons were meant to be used on the other nuclear powers, not nuclear free states.