Thousands of former Ambae residents remain in temporary settlements on nearby Santo, in often flimsy and leaky tents.
Kevin Kapalu from World Vision Vanuatu says thousands of people remaining on Santo are already suffering because of heavy rainfall.
But he said if there is not enough warning, a cyclone could be devastating for the evacuees.
"It will be another challenge if there's another cyclone within this cyclone period, then that will add another disaster on top, so they will become more vulnerable."