Moon-like craters found on the Marlborough Sounds seabed

The seabed of the Marlborough Sounds is being likened to the Sahara Desert and the surface of the moon.

NIWA researchers have spent 280 days painting a picture of what lies beneath the water. Marine geologist Helen Neil says what they found was a surprise.

"Sediment waves that are the size of the Sahara sand dunes formed because the largest tidal flow in New Zealand is flowing backwards and forwards through Cook Strait. Then you pop into the Sounds, think of craters on the moon - we can see holes in the sea floor."

They also found a long-lost passenger ship - the Hippolas, which sank in 1909 after it hit Walker Rock in rough weather.

"We've been able to map her location and the shape she is now on the sea floor. Fascinatingly, she's been there for over 100 years - she's now an artificial reef. She's got sea life growing on her."

An area the size of 80,000 rugby fields was mapped with 5.5 billion data points, collected by sending sound waves down from the bottom of NIWA vessels Ikatere and Rukuwai and measuring the reflections.

"The results of this survey deliver a quantum step in our knowledge of the Sounds' complex coast within NZ's extensive marine estate and provides a foundation for any further science in the region," said Dr Neil.

The information gathered will be used to produce new maps of the area, which were last updated in the 1940s.

"We have near-perfect coverage of the sea floor," says Dr Neil.

"Council now as a comprehensive data-rich appraisal of this vitally important coastal marine area," said Marlborough Mayor John Leggett. "This will help council in the sustainable management of coastal resources, and inform the community's growing desire for more comprehensive marine protection to maintain biodiversity."

Land Information Minister Eugenie Sage will visit the Sounds on Thursday ahead of a community meeting in Picton where the public will get to see the findings up close.