Ratu Epeli, the defence and national security minister, said the curfews were "to safeguard lives and property".
"We want to prevent criminal elements from taking advantage of the flood-hit areas," Epeli told the Fijilive website on Monday, adding that dusk to dawn curfews were in place in Ba, Nadi and Sigatoka towns.
Four people drowned in floodwaters and two were killed in a landslide in the western district of the main island of Viti Levu, Aisea Qumihajelo, the acting chief of disaster management, said.
Sugar cane crops were destroyed as rivers burst their banks and hundreds of foreign tourists were stranded in resorts after flights were disrupted in some airports and roads and bridges were severed or submerged.
Fiji's works ministry said the region now faced a health risk as people may drink water contaminated by the flooding.
Floodwaters that had reached as high as 3m in some areas had begun to recede on Monday in the northwestern towns of the main Viti Levu island, although some areas remained inaccessible and more seasonal storms were forecast in the next two days.