Radioactive material from Japan's tsunami-damaged Fukushima nuclear power plant has spread as far as Alaska, reports show.
But scientists and forecasters say global atmospheric conditions mean the spread of radiation is unlikely to reach New Zealand.
A 9.0-magnitude quake and ensuing tsunami on March 11 devastated Japan's northeastern Pacific coast, knocking out the plant's cooling systems and leaving it on the brink of a catastrophic meltdown.
High radiation levels have been detected in the seawater around the stricken plant, its operator Tokyo Electric Power Company has confirmed.
Nuclear material had also been detected in Alaska and North America - though at levels not damaging to human health.
The National Radiation Laboratory (NRL) and MetService today issued a statement saying those radioactive releases will almost definitely be confined to Japan and the Northern Hemisphere.
MetService Chief Forecaster Peter Kreft said atmospheric circulation patterns, which he compared to ocean currents, leave little or no chance of nuclear material reaching New Zealand.
His organisation and the NRL had both been fielding inquiries about the disaster.
"People are concerned for no particularly good reason.
"It's to be expected that increased levels of radioactive isotopes will be picked up somewhere. That's completely unsurprising.
"I'm not worried in the slightest."
He said it was important to get information on the spread of radiation from the NRL.
At the request of the the International Atomic Energy Agency, the World Meteorological Organisation (WMO) has activated an emergency environment response to the problems at Fukushima.
IAEA head Yukiya Amano yesterday told a 35-nation board meeting that - while the situation at Japan's crippled Fukushima Dai-ichi nuclear site remains serious - "we are starting to see some positive developments."
NRL Emergency Response leader Tony Cotterill said international agencies were monitoring the spread of radioactivity from the nuclear power plant using air monitoring stations in New Zealand and other countries.
There had so far been no report of radioactivity reaching a level that would be damaging to human health in countries outside Japan.