Gerald Hensley, former head of the Prime Minister's Office under Robert Muldoon and David Lange, said he was the ''hiss and boo figure'' for the 65 people at the debate at the University of Otago.
New Zealand was not safer being nuclear-free, he said.
''We did not leave Anzus to be safer, we left it because we were already safe.''
New Zealanders had lived in peace for 70 years and there was no ''plausible threat on the horizon''.
However, as Australia remained a member of Anzus, we were still under the Anzus umbrella because since World War 2, New Zealand and Australia had been considered ''one strategical entity''.
''A threat to one of these countries is inescapably a threat to the other. You could not threaten either New Zealand or Australia separately without the other immediately feeling an equal threat.''
Since the bomb was dropped in Hiroshima 68 years ago, no country could call itself nuclear-free, because every country lived with nuclear threat, Mr Hensley said.
But a worldwide ban of nuclear weapons would be possible only if there was a world government.
Prof Kevin Clements, the director of the University of Otago National Centre for Peace and Disarmament Studies, said nuclear weapons were an abomination.
''We used to joke in the 60s, 70s and 80s about how to learn to stop worrying and to learn to love the bomb. I never got round to that.
"How do you snuggle up to a bomb of this magnitude?
"Even with a low probability of the use of nuclear weapons, the mortality and ''lethal risk'' was enormous and nuclear weapons were nearly used in the Cuban missile crisis.
''We've gone to the brink.''
New Zealand's seclusion allowed its nuclear-free stance.
''We can stand up because it means we can say things larger countries in more central parts of the World can't ... In the 1980s was a moment New Zealanders said we wish to be nuclear-free.''
The debate was part of the 68th Hiroshima Day commemorations, reflecting on the nuclear bombing of Hiroshima on August 6, 1945.
