The slow-moving storm hammered the island for three days, causing major problems in a country where 370,000 people are still living in temporary shelters because of the devastating 2010 earthquake, The Christian Science Monitor reported.
Food prices were already rising before the storm hit, and officials are worried that damage to the water supply will lead to another cholera epidemic.
Peleg Charles, a spokesman for the charity Oxfam in Port-au-Prince, said Tuesday the worst-hit areas appear to have been in the south and are still inaccessible.
"In and around the city of Les Cayes, there is flooding everywhere," he told the Monitor. "It's disastrous ... But we won't know how bad it really is for several days."
At least 52 people are believed to have been killed by the storm in Haiti and 15 are missing.
Charles said 150 new cholera cases have been reported since Sandy departed. At least 7,000 people have been killed by the disease since the earthquake.
In late August, Hurricane Isaac hit Haiti, killing more than 20 people.