Green Party co-leader Jeanette Fitzsimons says there's nothing the Government can do to stop ships carrying reprocessed nuclear fuel travelling in New Zealand waters.
Greenpeace says two vessels carrying 1.8 tonnes of mixed oxide have left France for Japan and could travel near New Zealand.
It claims the shipments are the largest-ever of reprocessed uranium and plutonium.
Ms Fitzsimons says the Government cannot force the vessels to stay out of New Zealand waters, but should demand that they stay out of New Zealand's exclusive economic zone.
Anti-nuclear legislation can only keep them out of New Zealand harbours.
Environment Minister Nick Smith says he will send a strong message to the Governments of Japan and France that the vessels keep to the high seas.