Newly uncovered data from radioactive sheep has provided "robust" evidence that a "double flash" detected nearly 39 years ago from a remote island group was a nuclear explosion.
The Minister for Arms Control, Winston Peters, has asked officials to brief him on the findings after they were highlighted by Wellington health researcher Professor Nick Wilson.
Since soon after the flash was detected by optical instruments from a US "Vela" satellite high above Earth in September 1979, there has been debate about whether it was a nuclear explosion - and speculation that it was a weapon test by Israel. Some believe it was simply a meteoroid hitting the satellite.
The flash was located in the vicinity of the tiny Marion and Prince Edward islands in the South Indian Ocean, about halfway between Africa and Antarctica.
"A new publication sheds further light on the Vela Incident of 1979," said Wilson, of Otago University at Wellington, "and adds to the evidence base that this was an illegal nuclear weapons test, very likely to have been conducted by Israel with assistance from the apartheid regime in South Africa".
"This would have been a violation of the Limited Test Ban Treaty …," said Wilson, an epidemiologist and member of the Australia-based Medical Association for the Prevention of War.
He urged the Government to ask Israel to comment; ask the United Nations to investigate; help finance South Africa to look into the apartheid regime's involvement; and ask the United States to release classified documents about the Vela incident.
Israel has dismissed the claim that it was responsible for a nuclear explosion in the South Indian Ocean in 1979.
The new publication is a paper in the journal Science & Global Security by Christopher Wright of the Australian Defence Force Academy and Lars-Erik De Geer, who has retired from the Swedish Defence Research Agency.
They analysed previously unpublished results of radiation testing at a US lab of thyroid organs from sheep in southeastern Australia. Some of the material had been released under US freedom-of-information laws.
"By 1979, it had long been established that thyroid glands of grazing animals, and especially sheep, efficiently concentrate radioactive iodine-131 from atmospheric nuclear weapon tests," the researchers wrote.
Iodine-131 is an unstable, radioactive form of the element iodine.
Thyroid samples from sheep slaughtered in Melbourne were regularly sent to the US for testing - monthly in 1979 - from the 1950s to the 1980s.
The journal article said test results from different thyroid samples of sheep slaughtered on three different dates showed "two signature gamma-ray emission lines of iodine-131".
The sheep had been grazing in an area hit by rain four days after the optical flash was detected. They were in an area that a weather study showed was in the downwind path from the suspected explosion site. The plume would have passed south of New Zealand.
The quantity of radiation from the iodine-131 fallout was tiny, but the "signal-to-noise ratio" was high.
The sheep data added a third pillar of evidence that the Vela incident was a nuclear explosion, the authors said. The other two pillars were the flash and detection of a "hydroacoustic signal" by underwater listening devices.
"... analysts have previously argued that the optical and hydroacoustic signals are definitive indicators for a nuclear test, while the iodine-131 detections provide robust and credible evidence for a nuclear fission event …"
Commenting on the findings, US nuclear weapons expert Leonard Weiss of Stanford University says on the online Bulletin of the Atomic Scientists that the "important" new evidence "removes virtually all doubt" that the flash was a small-yield nuclear explosion.
He added that there was "growing circumstantial evidence" that it was conducted by Israel.
"Israel was the only country that had the technical ability and policy motivation to carry out such a clandestine test …"
The official position of Israel is to neither confirm nor deny the existence of a nuclear programme, although the Guardian has described its existence as an open secret. The country's former Knesset Speaker Avrum Burg told a conference in 2013 that "Israel has nuclear and chemical weapons" and called for public discussion.
Israel's Ambassador to New Zealand, Itzhak Gerberg, told the Herald, when asked if Israel was responsible for the explosion: "Simply a ridiculous assumption that does not hold water."
Peters said: "The issue raised in the correspondence is scientifically and technically complex, and examines something which may have taken place almost 40 years ago.
"I will be happy to respond once officials and technical experts have had an opportunity to fully assess the material and provide a briefing to me."