The Greens in French Polynesia are urging Paris to give information about the risks of Moruroa atoll collapsing.
The former nuclear weapons test site is a no-go zone, which France has kept despite promising to return it to French Polynesia after the end of the testing regime in 1996.
France is refusing access to independent monitors while saying the chance of a collapse is practically nil.
A spokesperson for the Greens in Tahiti, Olivier Champion, says France has a record of lying about the tests and their effects, and many fear a Fukushima-like disaster is possible.
"Transparency, of course, and another thing, a plan what to do in case of... if the reef barrier of Moruroa and Fangataufa is collapsing, what is the plan? What can we do? How can we react?"
Olivier Champion says the chain of command via France is so long that a possible tsunami would hit nearby atolls before a local warning is issued.
He says nuclear contamination could imperil the livelihood of vast areas of the Pacific.