Scientists are trying to work out the risk of a major earthquake along one of New Zealand's most dangerous fault lines.

More than 30 seismometers on the seafloor off Poverty Bay will measure quake activity and tsunami risk in the Hikurangi subduction zone.

Seismologist Bill Fry says subduction zones are responsible for the world's biggest shakes.

He says an event on this fault would produce the largest shake in the country - up to a magnitude 9.0.