"There are only two challenges that threaten life on a planetary scale: global warming and nuclear weapons. They're both caused by machines we've made and they're both reversible and they both require new policies and leaders to change course," Ploughshares Fund president Joe Cirincione said on Wednesday.
"If we don't, a nuclear disaster is going to happen. We've been lucky over the past 60 years, but that luck's not going to hold. And when it happens, it will be the biggest catastrophe in human history."
Mr Cirincione (60) was in Dunedin on a US State-sponsored visit to talk at the University of Otago.
The Ploughshares Fund is a public grant-making foundation which focuses on nuclear weapons policy and conflict resolution.
Previous nuclear policy was doomed to failure, he said.
"The status quo is an illusion. It's unsustainable. Going to war in Iraq was justified as a solution to stop hostile states getting the nuclear bomb. It was meant to be a warning that you go up against the US and develop a nuclear programme and you see what happens. And it backfired badly.
"You can't go around the world playing nuclear whackamole. Trying to find a military solution to the problem, like [former-US president George W.] Bush did, only makes the problem worse.
"Iran and North Korea have made more progress in the past six years than in the previous 16. They weren't deterred. Countries like Saudi Arabia, Egypt and Turkey have all started the decade-long task to build the bomb.
"We either stop Iran or there will be three, four, five nuclear powers in the Middle East. With all the territory and religious disputes there, that's a recipe for nuclear war."
The Washington-based advocate said New Zealand's 1987 nuclear-free zone legislation had proved prescient.
"New Zealand was right and the rest of the world is now recognising the wisdom of New Zealand's decision. The only way to stop others getting nuclear weapons is to stop ours. As long as any nation has nuclear weapons, other countries will want them. We want to lock up all the nuclear weapons over the next four years. That's the programme.
"We're at the nuclear tipping point and it's a unique moment in nuclear policy. There's a fundamental shift in thinking and a new, emerging consensus. People are getting serious about eliminating nuclear weapons.
"I'm encouraged by Kevin Rudd's appointment as Australian foreign minister and I'm encouraged by John Key's influence with the Obama Administration. The answer to this problem has to be part of a regional and global solution. A new realism."
Mr Cirincione was an adviser on nuclear matters to Barack Obama during his 2008 presidential campaign and served as vice-president for national security and international policy at the Centre for American Progress.
In 2005, the World Affairs Councils of America named him one of the 500 most influential people in shaping American foreign policy.
