Scientists have discovered there is no connection between main fault lines in the North Island and South Island, so any big earthquake will be contained to one island.
NIWA used high-resolution images of the layers below the seabed to assess the prehistoric earthquake activity of some of the faults.
Scientists mapped 20 active faults in the Cook Strait region capable of generating earthquakes of measuring at least 7.0 on the Richter scale.
They found none connect between the two islands.
NIWA says the research will have a big impact on assessing earthquake risk and will be passed on to emergency managers, regional councils and insurers.