Only 12 per cent voted against the assembly and the remaining ballots were void or left blank, according to the poll, which had a margin of error of 2 percentage points.
"The 'yes' vote had an overwhelming majority," Correa said at a brief news conference in the coastal city of Guayaquil on Sunday. "Thank you Ecuadoreans."
Correa, who has scared foreign investors with threats to stop making debt payments, staked his political career on the vote, pledging to resign if he failed to win emphatically.
Voting is mandatory for Ecuadoreans and Sunday's turnout appeared as high as for last year's presidential election.
Ecuador, the world's top banana exporter, has had eight presidents in a decade, three of them toppled in popular and congressional unrest.
Congress has been blamed for causing instability and being corrupt, but critics of the president say he is consolidating power and becoming a dictator.
Mariana Sanchez, Al Jazeera's correspondent in Quito, said the exit poll showed how much Ecuadoreans are looking for change.
She said Correa understood that he would not be able to govern effectively with a powerful congress opposed to him, so he went to his power base: the people.
It is a tactic similar to that used by other leftist presidents in the region.
Hugo Chavez in Venezuela and Evo Morales in Bolivia also called referendums soon after taking office, seeking to emasculate traditional parties.
More than half of Ecuador's congressmen were fired last month after opposing Correa's plans for the referendum.
They fought with police to get back into the chamber but were ultimately sidelined when congress convened with substitute members, as allowed by the country's electoral court.
If confirmed, a clear win is expected to bolster Correa's mandate and allow him to push ahead with initiatives such as ending the lease on a US military base, renegotiating oil deals and restructuring the national debt.