The government ordered the evacuation of Ambae's population of about 10,000 people three months ago because of volcanic activity.
Oxfam's Sandra Uwantege Hart said some evacuees were living on Maewo and Santo in tents and shelters made from tarpaulin and sticks.
"The cyclone season has already begun. So the level of concern is extremely high amongst humanitarian actors here around how quickly can safe shelters be built compared to how quickly the cyclone season is unravelling."
Sandra Uwantege Hart said without assistance from the government, some Ambaeans on Santo had found carpenters to help them build houses.
She said on Maewo the government was helping communities clear land for permanent structures.
Photo Vanuatu Red Cross.