World weather patterns will shield New Zealand from radiation spewed out of the stricken nuclear power station in Japan, experts here say.

Traces of radioactive material from Fukushima in northern Japan have reached most parts of the Northern Hemisphere.

But New Zealand scientists said yesterday that even in the event of a complete meltdown at the plant, the South Pacific was shielded from fallout by global weather patterns.

Radiation from the tsunami-damaged plant has been found in the United States, Canada, Iceland, Germany, China, and South Korea, but not in doses posing a health hazard.

Prevailing winds push microscopic particles east of Japan at about 80km/h. The material took about a week to cross the Pacific.

The detection of radiation in distant countries was largely due to the hyper-sensitivity of modern sensors, as opposed to large amounts of material being carried by the wind.

GNS Science senior scientist Bernard Barry said even if the partial breakdown of the reactors worsened, particles would not travel deep into the Southern Hemisphere.

Unlike Chernobyl, the Fukushima event did not have an explosion to propel the nuclear material into the stratosphere, where it could spread quickly.

Dr Barry said: "Chernobyl was a big block of graphite, like hundreds of tonnes of barbecue charcoal, which burned for 10 days, vapourising stuff and sending it into the atmosphere. There's not that type of material in Japan, it's a completely different reactor."

Even if the nuclear particles were projected above the lower atmosphere, a "barrier" at the equator prevents the transfer of most northern weather into our territory.

In what is known as the coriolis effect, a force generated by the rotation of the earth nudges wind into rotating anti-clockwise north of the the equator and clockwise south of the equator.

Most wind is deflected at the equator, and there is minimal transfer of weather between the hemispheres.

Any radioactive particles which did creep into the South Pacific would be diluted and slow-moving.

"If [radiation] did ever get here, it would take months, maybe years, and only be in scientists' interest," said National Radiation Laboratory spokesman Peter Abernethy.

The NRL, an arm of the Ministry of Health, has radiation monitoring stations at Kaitaia and the Chatham Islands.

It can also collect readings from stations around the Pacific, many of which were developed to measure fallout from testing of nuclear weapons in the Pacific in the 1960s and 70s.

The monitoring stations can detect a tiny fraction of a millisievert (mSv), a radiation unit. A dose of about 1000 mSv, which the workers at Fukushima could be exposed to, will make someone sick. Everyone is exposed to 2 mSv a year through naturally occurring radiation.

Japanese authorities yesterday said small amounts of plutonium - a byproduct of atomic reactions - had seeped into soil at the tsunami-damaged plant in northern Japan.

Radiation which has seeped into the sea is unlikely to reach New Zealand, because it is dispersed widely by ocean currents.

- Additional reporting: Agencies