Radioactive isotopes found in Australian sheep have added credence to the theory that Israel conducted an illegal nuclear test over the Indian Ocean 39 years ago.

The findings, published in a new study for Science and Global Security, shed intriguing new light on the mysterious Vela Incident, as it is known, of September 22, 1979.

At 12.53am GMT on the date, the US satellite Vela 6911 detected the 'double flash' characteristic of a nuclear explosion in the southern Indian Ocean, near the Prince Edward Islands about halfway between Africa and Antarctica.

Advisors to then-President Jimmy Carter rushed to brief him on the incident, and security officials immediately speculated that the event was an Israeli nuclear test conducted in cooperation with apartheid South Africa, Carter wrote in his memoirs.

This map published with the new study extrapolates the likely nuclear plume from a detonation near the Prince Edward Island. The areas in purple show rainfall that could have carried fallout

However, an official US government panel convened to study the matter delivered an equivocal finding that downplayed the likelihood of a nuclear explosion.

Israel, whose presumed nuclear arsenal is considered an open secret by many, has steadfastly refuse to confirm or deny whether it has a nuclear program.

Now, the new study by Christopher Wright of the Australian Defence Force Academy and retired Swedish Defence Research Agency nuclear physicist Lars-Erik De Geer, offers new clues.

The researchers reveal the discovery of iodine-131 in the thyroids of some Australian sheep in October and November of 1979. The thyroids were sent to the US for analysis at the time, but the results were never made public.

The researchers write that the isotope levels 'would be consistent with them having grazed in the path of a potential radioactive fallout plume from a 22 September low-yield nuclear test in the Southern Indian Ocean.'

The study reveals that the isotope iodine-131 in the thyroids of some Australian sheep in October and November of 1979. Pictured: Sheep in New South Wales earlier this month

The findings include analysis of weather patterns that suggest the fallout plume from a nuclear explosion would have looped over parts of Australia.

As well, the study analyzes declassified descriptions of an underwater sound wave detected by US listening posts that correlated with the double flash near the Prince Edward Islands, which are uninhabited except for a South African government research station.

The new study 'removes virtually all doubt that the 'flash' was a nuclear explosion,' Leonard Weiss wrote for the Bulletin of the Atomic Scientists.

'This strengthens previous analyses concluding that Israel likely carried out a nuclear test in violation of US law and the Limited Test Ban Treaty,' the nuclear nonproliferation expert wrote.

'Israel was the only country that had the technical ability and policy motivation to carry out such a clandestine test, which, according to some sources, was the last of several and was detected by the Vela satellite because of a sudden change in cloud cover,' Weiss wrote.

Israel for its part maintains a strategic silence on the question of a nuclear weapons arsenal.

Asked if Israel was responsible for the Vela Incident, Israel's Ambassador to New Zealand, Itzhak Gerberg, told the New Zealand Herald: 'Simply a ridiculous assumption that does not hold water.'

The Limited Test Ban Treaty went into force in 1963 and bans nuclear explosions in the atmosphere, in outer space, and under water, rendering legal only those nuclear tests performed underground.

Israel signed the treaty in 1963 and ratified it in 1964.