Officials are playing it down, saying it has been going on for ages. Turns out that uranium ore concentrate has been coming through our ports for 30 years, but only at the rate of one shipment per annum.

However, in the past few months permits have been granted for almost weekly visits. Hands up who knew?

Environment Minister Nick Smith only found out a few months ago. Opposition leader Phil Goff, Minister of Foreign Affairs and Trade (MFAT) and Defence in the last Labour government, and Goff's disarmament spokesman, Phil Twyford, didn't know until recently, nor did the Greens, or environmental watchdog, Friends of the Earth.

Georgina te Heuheu, minister for Disarmament and Arms Control, was told last year but made no announcement.

Not that it was a secret. It was just that, until recently, it was very unlikely anyone would stumble across the information. That is because it appears that until last year the shipments didn't require a permit.

The National Radiation Laboratory administers importations of radioactive material, but because these shipments are transiting they don't require its consent. It does, however, monitor the shipments.

MFAT started vetting yellowcake shipments only last year under the Customs and Excise Act which requires MFAT consent to transit strategic goods, such as yellowcake .

But even now, it doesn't make details about the shipments public, for commercial and confidence reasons. It takes this approach, a spokesman said, to encourage the shipper to fulfil its legal obligation to disclose whether the cargo "is intended for or may have any of the ... prohibited end-uses". Read here, blowing stuff up.

In that spirit, it would not provide the Herald with details of the shipments this week.

That policy was made redundant, however, by the approach of another arm of government - the Environmental Risk Management Authority (Erma), whose consents for the shipping can be found on its website.

Erma, too, is a Johnny-come-lately to a vetting role. A spokeswoman told the Herald it only became aware late last year that the shipments fell under the hazardous substances section of the HSNO Act 1996 which Erma administers.

It was references to uranium shipments on Erma's site which recently caught the eye of Friends of the Earth and consequently the media.

At our request, the National Radiation Laboratory dug through old reports trying to find out when the shipments started, who authorised them and whether the government of the day gave the okay.

They found this reference in the Laboratory's official review document of 1976-1980, which indicates ships loaded with uranium began stopping over at least three decades ago: "Shipments of uranium oxide ores through New Zealand ports initially gave rise to some apprehension on the part of shipping organisations and trade union, although in fact the ores present a negligible radiation hazard."

The timing coincides with the passing of our Nuclear Free Zone, Disarmament, and Arms Control Act 1987, but no one seems to know about the role of the government at the time. "I've asked our longest-serving staff and they don't know," says Cris Ardouin, a senior advisor with the Laboratory.

Erma documentation shows that all three companies mining uranium in Australia got permission this year to regularly route their product via New Zealand until the end of 2014.

The companies are Energy Resources of Australia (a subsidiary of British-Australian group Rio Tinto) which owns the Ranger Mine near Darwin; BHP Billiton, owner of the Olympic Mine; and Heathgate Resources, owner of the Beverley Mine (both in South Australia).

Ships can stay in port for up to 20 days. Each carries hundreds of tonnes. Energy Resources of Australia, for instance, told Erma its shipments could be as much as 750 tonnes.

The yellowcake must remain in the ship's hull while in port.

Though yellowcake has undergone processing from the material mined, its radioactivity is unchanged and remains low. The National Radiation Laboratory considers it to pose little risk to the public.

It is, however, toxic if inhaled or swallowed and prolonged or repeated exposure could cause severe damage to the liver and lungs.

Most concern about the shipments is not about the health risk to the New Zealand public but whether some of its by-product, depleted uranium, may end up in weapons.

That, and how allowing the shipments stacks up against New Zealand's nuclear-free reputation.

The destination of shipments by Energy Resources of Australia, is listed as Honeywell's plant in Metropolis, Illinois. That plant converts uranium for nuclear fuel and electric power stations.

Honeywell reportedly said this week that all of its processed uranium was used in the production of nuclear power for civilian use and there was no military usage.

However, the US Environmental Protection Agency lists two Honeywell plants (both in Minnesota) as "depleted uranium manufacturing and testing facilities".

Another of the yellowcake exporters, Heathgate, is a subsidiary of General Atomics, best known for making the predator drones used in Afghanistan and Iraq.

It is owned and chaired by reclusive American billionaire Neal Blue, an influential lobbyist of those in power. The Australian Financial Review named him in its Covert Power list of the major string-pullers in the country last year and the US Centre for Public Integrity rated his company as the biggest corporate sponsor of travel for US Congress officials in 2000.

Blue was accused in a civil law suit of price-fixing regarding uranium from its Australian mine, and a General Atomics' spy was caught in the same year having infiltrated Australian environment groups.

Smith seems happy with what he described (in answer to a question in parliament Thursday from Green MP Gareth Hughes) as the Australian government's "strict criteria that it will allow the export of uranium ore only for peaceful purposes".

The bottom line, contends Friends of the Earth spokesman Bob Tait, is that there is no way of knowing for sure where the depleted uranium produced in processing ends up. Because of its high density, depleted uranium is used in armour-piecing missiles.

New Zealand imports some of the end products of uranium, such as for use in medicine.

There was no plan to stop such shipments, which Smith said did not undermine New Zealand's "proud nuclear-free history".

On that Robert White begs to differ. White, a retired nuclear physicist and co-founder of the Centre for Peace Studies at Auckland University and Scientists Against Nuclear Arms says we will do just that by allowing transit of radioactive material associated with the nuclear power industry, "which in turn is closely linked by many researchers to nuclear weapons programmes".

White is calling on te Heuheu, as disarmament minister, to ban the shipments which he says breach the Marine Pollution Act which makes it an offence "to store radioactive waste".

But a spokesperson says the minister is satisfied the transit of Australian yellowcake complied with all relevant domestic and international laws and as it was specifically for use only for civil nuclear purposes "poses no credible threat to New Zealand interests nor to our longstanding and respected position concerning nuclear weapons".

One question left unexplained is why the sudden leap in frequency of shipments via New Zealand?

None of the three mining companies was prepared to answer that or any other of the Herald's questions.

It may be that the route is considered safe. Great caution was taken on the shipment of 550 tonnes of yellowcake from Iraq in 2008. Described as the "last remnant of Saddam Hussein's nuclear programme" it was taken out of the country on 36 military flights, the small loads due to fears the cache might reach insurgents or smugglers crossing to Iran to aid its nuclear ambitions.

The cargo
Uranium oxide or yellowcake from Australian mines, shipped in powder form and sealed inside steel drums packed in containers.

The ports
Vessels carrying yellowcake have been stopping at Auckland, Tauranga, Napier and Nelson for three decades.

The dangers
Yellowcake can cause lung damage if inhaled. The material is used by the nuclear power industry, but questions remain about links to the military use of nuclear material.