Princeton, N.J.-based Climate Central said that future sewage leaks are a major risk because rising sea levels can make coastal flooding more severe.
The group, which compiled data from state agencies and treatment plant operators, did not look at the specific environmental or public health impact of the sewage overflows after Sandy, which struck in late October. But it said that bacteria in sewage can spread water-borne illnesses and have a particularly bad effect on shellfish.
In New Jersey, officials spent months monitoring shellfish beds for contamination and reopened the last of them in mid-April, said Larry Hajna, a spokesman for the state Department of Environmental Protection.
The collective overflows - almost all in New York and New Jersey and due to storm surges - would be enough to cover New York City's Central Park with a pile of sewage 41 feet high, Climate Central said.
About one-third of the sewage was not treated at all and the rest was not completely treated.
The group said the estimated cost of repairing damage to sewage treatment plants after Sandy is nearly $2 billion in New York and $2.7 billion in New Jersey.
Alyson Kenward, the lead author of the report, said that treatment facilities should raise power generators and other critical components to minimize future overflows. She also said separating sewage lines from storm water overflow pipes can also help, though that's an expensive undertaking.
"These facilities do by design have to be relatively close to the water," she said. "They are always going to be vulnerable to coastal flooding."
Kenward said quick work of treatment plants saved billions more gallons of untreated sewage from entering waterways.