Researchers have found more of the South Island could be in line for a much larger quake than anyone thought.
It has always been assumed the country's largest fault, the Alpine Fault, was a near-vertical crack.
But researchers have found it curves under much of the South Island.
Associate Professor Simon Lamb from Victoria University said instead of the two plates sliding past each other, they turned horizontal, and one was sitting on top of the other.
"The area that we're talking about actually extends from Pukaki in the Mackenzie Country down to Wakatipu.
"That's really where the fault is at its widest, and actually it becomes wider as you go south."
The width extended to about 100km, he said.