But Professor Wyn Bowen, from the Department of War Studies at London's King's College, warns that no country will be exempt if Obama's fears are realised.
Professor Bowen will give a talk on nuclear security at Auckland University next week. He was a weapons inspector in Iraq in 1997-1998 and is a consultant for the International Atomic Energy Agency.
"The key issue here is that a nuclear terrorist event, wherever it occurred in the world, would have knock-on effects for everybody - economically, politically.
"Even if you had an improvised nuclear device smuggled by a terrorist organisation anywhere in the world, even if it was not detonated the response to that would be akin to 9/11. Because it would demonstrate that terrorist organisations were capable of launching such attacks."
He said more attention was paid to nuclear safety because there had already been disasters - Chernobyl, Three Mile Island and, most recently, Fukushima.
Less attention was paid to nuclear security because there had not yet been any terrorist attacks with nuclear material although there were cases of trafficking.
He sets out four possibilities for nuclear terrorism - including the worst but unlikely scenarios of terrorists getting their hands on a nuclear weapon or making a bomb from highly enriched uranium or plutonium.
More likely was a "sabotage attack" on a nuclear facility such as a power plant to release radioactivity.
"Fukushima is sometimes quite a good way to illustrate what a spectacular terrorist attack could potentially achieve."
The fourth possibility - of radioactive material being used in a 'dirty bomb' - was the most likely because of the prevalence of radioactive isotopes for medical and industrial use.
The Prime Minister agreed the issue should not be underestimated by New Zealand, despite its relative distance.
"I don't think there's any doubt if a rogue state or a rogue entity deployed some sort of nuclear weapon, however large or small, that would incite significant panic and fear. We find it hard to imagine someone using nuclear terrorism, but nonetheless we can't rule that out," he said.
Auckland University nuclear security expert Tania Ogilvy-White has questioned why New Zealand is yet to ratify the two international instruments that the summit identified as critical for nuclear security.
The first is the International Convention for the Suppression of Acts of Nuclear Terrorism - which requires states to have domestic laws setting out penalties for the unlawful possession of nuclear and radiological material.
The second is the Amendment to the Conventions for the Physical Protection of Nuclear Materials which requires safe transport of those materials.
The ratification of those is expected to be among New Zealand's commitments at the summit.
Mr Key said the delay was partly because of the local law changes required before they can be ratified.
In 2010-2011, there were 172 "incidents" reported to the IAEA, including 14 cases of unauthorised possession or attempts to smuggle or sell nuclear and radiological materials.
In five cases, the nuclear material was highly enriched uranium.
Disarmament campaigner Alyn Ware feared Mr Key was wasting the opportunity Obama had given him by inviting him to the summit.
Ware, who founded the Parliamentarians for Nuclear Non-proliferation and Disarmament network, said it was effectively a nod to New Zealand's stance.
"But that appeared to be lost on the Prime Minister."
It is a view echoed by Centre for Strategic Studies director Robert Ayson.
While New Zealand was once at the forefront of international disarmament issues, that appeared to have waned.
"One of the reasons Mr Key was invited in 2010 was because of that legacy. This is an opportunity, because Obama has made deep nuclear cuts quite a priority.
"That has not been overwhelmingly popular in the United States, particularly among the Republicans, but he is on a bit of a charge over this and for a country like NZ, given its past record, this is an opportunity.
"The question is are we really doing what we can to encourage this if it is a New Zealand priority still?"
The summit has already had success. Soon after the Washington forum Russia and the US agreed to start reducing nuclear weapons. A stocktake on the commitments two years ago showed 80 per cent were completed.
The US has reported that Romania, Libya, Turkey, Chile and Serbia have cleared stockpiles of weapons-grade uranium.
For New Zealand it is a rare opportunity for the Prime Minister to be in the same room as major world leaders.
With the US elections looming in November, this autumn in Seoul could be the last chance for Mr Key to make the most of his status as the unofficial mascot.
Rogue elephants in room
When world leaders arrive in Seoul for the Nuclear Security Summit on Monday there will be two elephants in the room: North Korea and Iran.
The summit agenda does not include discussion of either trouble spot. But there is little doubt that North Korea in particular will dominate talks on the sidelines.
North Korea made sure of that when it announced plans to launch a rocket to put a satellite into space in April.
The statement came soon after it reached an agreement with the US to halt ballistic missile testing, suspend its uranium enrichment and allow weapons inspectors back in return for food aid.
President Obama has made it clear he will raise the issue with China's Premier Wen Jiabao in a bid to get Beijing to use its relatively good relationship with North Korea.
Nuclear proliferation expert Professor Wyn Bowen said the plan was "classical brinksmanship".
Said Bowen: "They're effectively tweaking the nose of the international community by saying 'we're not going to launch any military rockets - we're going to launch a space rocket as a celebration'."
"This is classic North Korea."
Deputy political editor Claire Trevett will cover the Nuclear Security Summit for the Herald.
