Flint was deemed the "most expensive" water provider of the nation's 500 largest community water systems in January 2015, and its residents paid $864.32 yearly for 60,000 gallons of water, the Food & Water Watch group said.
That rate was almost three times national average of $316.20 for government utilities, the group said.
The group advocates publicly owned water systems, which it says provide the most equitable and affordable service, and the group stands up "to corporations that puts profits before people," it says. Flint's water system is publicly owned.
The next highest water provider was the publicly owned service in Bellevue, Washington, where residents paid $855.25 annually, the group said.
Flint residents didn't have to pay the highest water bill for too long because in August 2015, a state judge ordered the city to roll back water and sewer rates by 35% and end a monthly service charge that generated $900,000 systemwide, according to Food & Water Watch and and the attorney for residents who filed two suits.
The state judge also ordered two sides in a lawsuit to discuss how the city must repay its water and sewer fund by $15.7 million, said the attorney, Valdemar L. Washington, also a Flint resident and a retired state judge.
The nonprofit group's finding was no surprise to Washington. He is the attorney behind two pending lawsuits that were filed after residents saw water and sewer rates increase by double-digit percentages in 2011 and 2012, he said.
"I didn't need a study to tell me I was paying the highest bills," he said. "All I had to do was look at my water bills."
"What prompted me to take action was one month's bill," which amounted to $600, Washington said.
"I said there was something really wrong with that picture, and that's when I sued."
"Even in Phoenix, where there's a desert, they don't pay as much as we do out here. And we're surrounded by the Great Lakes. What's going on?" Washington said.
Activists expressed outrage about last year's water bills in Flint, which is 57% black and 41% impoverished.
"It's completely insane for people to be paying so much who are getting frankly so little," said Ben Jealous, former president and CEO of the NAACP.
A spokeswoman for Michigan Gov. Rick Snyder said Flint was in a financial emergency about two years ago, when the state decided to save money by switching the city's water supply from Lake Huron to the Flint River.
But the river had such a notorious history that locals called it the "General Motors sewer."
Flint's drinking water later became contaminated with elevated levels of lead, and now the state is handing out bottled water and water filters to residents.
"The Flint City Council originally voted to move away from Detroit Water and Sewerage Department in large part because water rates for residents were unaffordable. Therefore the data in the report is not surprising," said Anna Heaton, a spokeswoman for the governor.
Flint officials declined to comment on the nonprofit group's findings: "Not at this time. We've been busy addressing some other matters today," city spokeswoman Kristin Moore said.
On Wednesday, Flint Mayor Karen Weaver said she wants the replacement of the city's lead pipes to begin next week so that the community can have clean water as soon as possible. She urged Snyder to pressure the state legislature to move immediately to approve funding for the first phase of a $55 million lead pipe replacement plan.
The governor also favored speedy pipe replacement, but it's part of multistep process.
"I don't know if I'd want to say next week or not, but we're talking a very short timeline to start having some pipes replaced in the community," the governor said. "I don't think it's inconsistent with what the mayor is saying. Again, people may have differences in timelines, but her goal is to get pipes replaced. Well, that is one of my goals, but it's one of the steps in this larger process."
Snyder announced on Thursday that Flint will get an additional $2 million from the state to help pay to replace the city's water infrastructure. The money will cover the costs for replacing water lines for several hundred homes, according to Weaver's plan.