In the early seventies? There were more than enough to go around, and Australia hosts US facilities. Pine Gap for example would have been an early target along with Holt station and Nurrungar. Deliberate city strikes depend on a switch to Countervalue targets. Plenty of Soviet subs in the Pacific....If you take into account the unreliability of the strategic delivery systems, Australia would be largely immune. We are simply too far away and unimportant for the Soviets to waste warheads on. They have much more important targets to strike in the US and Europe. Our cities are too far apart and would not be of interest to them...