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."
Mr Kapalu said some of the materials for homes have already been damaged by use during 2015's Cyclone Pam.
Mr Kapulu said nearly six months after the mass-evacuation, thousands of people were still vulnerable and stressed out.
Many who have stayed in Santo have bought land there, but remain in shelters because they don't have the finances to build new homes.
He said nearly six months after the mass-evacuation from Ambae, thousands of people are still vulnerable and stressed out.
"If there's a big ran, amount of rainfall, then their tents will actually get flooded and be up all night," he said.
"And their beddings, their stuffs, their clothes will actually be wet."
Another World Vision Vanuatu worker, Joanna Halili, said many evacuation shelters are increasingly overcrowded.
"We have extra people coming in and it's quite a stress for the family, because we have like another 10 people on top of another five, then it will be like 10 and there will only be like two people having job, then using only one toilet."