The death toll from Cyclone Yasa, a top category-five storm that made landfall on Thursday, rose to four, authorities said.
Aerial assessment showed that Bua province on the northern island of Vanua Levu suffered 70% damage and destruction, while the small island of Kia was completely destroyed, Fiji Red Cross Director General Ilisapeci Rokotunidau said.
"It's like a ground zero, there's nothing left," Rokotunidau told Reuters.
While the largest island of Viti Levu - where about three-quarters of Fijians live - was spared the worst, Vanua Levu and other small islands were in dire need of fresh water and shelter.
Disrupted communications and poor weather were hampering the damage assessment effort but Red Cross volunteers were assisting those in need, Rokotunidau said.
There were concerns about the spread of diseases.
Australia is sending air force aircraft to help with surveillance of storm-affected areas and has offered further assistance to Fiji if needed.