GNS Science said staff and volunteers from the Forest Lifeforce Restoration Trust discovered two boulders containing the fossils of large vertebrae while assessing a property in the Maungataniwha Native Forest in March.
This was also the site of the first dinosaur fossil found in New Zealand, by renowned New Zealand palaeontologist Joan Wiffen and her team in 1975.
National paleontological collection manager Marianna Terezow has tentatively identified two fossil vertebrae elasmosaurus, an enormous marine reptile which could reach up to 14 metres long with a long, snake-like neck, and lived more than 80 million years ago.
Work is ongoing at GNS in Wellington to identify the fossils.
Trust forest manager Pete Shaw was an experienced fossil hunter.
"If any one place is the epicentre of New Zealand palaeontology then Maungataniwha, and particularly the Mangahouanga Stream, is probably it," he said.
The changes to the landscape post-cyclone were huge.
"It's like a giant has walked down the stream-bed, kicking at rocks and boulders as if they were pebbles and turning everything over as he goes."
The rocks containing the fossils were uncovered by the raging rivers, swollen by heavy rain, and the pressure of millions of cubic metres of rainwater scoured riverbeds and overturned boulders.
