It has faced criticism for its tardiness in moving to ratify the convention, although it means little in practical terms to New Zealand because of its lack of nuclear materials.
Mr Key defended the delay in ratifying the instruments, saying other countries were also yet to do so including Canada and Australia.
However, since the 2010 Summit 12 other countries have ratified the convention - including the United Kingdom and Australia has now passed the domestic laws enabling it to.
New Zealand has taken some steps to ratifying it by imposing criminal penalties for acts of nuclear terrorism in the Terrorism Suppression Act, but it had not yet updated its radiation safety legislation. That was expected to happen within the next year.
The other is an amendment to the Convention for the Physical Protection of Nuclear Materials which requires measures to protect transport of nuclear and radiological material.
New Zealand does not have nuclear material - highly enriched uranium and plutonium - but it does have some radioactive material, used mainly for medical purposes.
Mr Key said such measures were important to send a signal to other countries.
New Zealand was the only completely nuclear free country among those at the Summit, which was instigated by President Obama in 2010 as part of his broader vision for a nuclear weapons free world.
"We're the only country that doesn't have nuclear power, nuclear materials or nuclear weapons and the reason for that is our long-held anti-nuclear stance. The great irony is that one of the policies that led to a very frosty relationship with the United States all those years ago is now being held up as something that is both useful and making a valuable contribution in that regard. It's a positive - and it's a coming of time."
The 54 leaders attending include President Obama, Russia President Dmitry Medvedev, China's president Hu Jintao, India Prime Minister Manmohan Singh, Pakistan Prime Minister Yousaf Raza Gilani, and Australia Prime Minister Julia Gillard.
Most of the countries have nuclear materials - such as high enriched uranium or plutonium - either for nuclear power or nuclear weapons.
The Summit is aimed at ensuring old stockpiles are cleared out and other materials are stored securely so they can not be stolen or are not left vulnerable to sabotage attacks by terrorists.