Those who have been making the court-ordered payments will get some money back, but their former spouses who are receiving support will see their payments reduced.
The provincial agency blames an administrative error for the fact it has been charging some people too much for 15 years by adding annual cost-of-living adjustments to the support payments, even though those adjustments were eliminated in 1997.
"Several months ago they started to find this trend and did a thorough investigation and reported to me before Christmas that they had discovered these 1,700 cases," said Social Services Minister John Milloy.
"They’re still continuing to look through cases that are closed."
The 1,700 cases are only a small fraction of the total handled by the Family Responsibility Office, and in each case the mistakes were made because the file had been re-opened by the courts.
"What happens is occasionally these files are re-opened because of a new updated court order, and FRO applied the old rules instead of the new rules," said Milloy.
The extra charges amounted to $5.3 million, or about $75 per month for the average client affected by the mistake.
All those affected will see their court-ordered support payments lowered and the overpayment will be credited to their account or reimbursed.
People who received more than their court-ordered amount will have future payments reduced to correct the error, but they will not have to pay back any money because of the agency's mistake.
"We’re trying to be transparent about having found this error but also to alert individuals who have had dealings with FRO," said Milloy.
"We want to make sure people are aware this review is going on and there are people who may see changes in their cheques."
The Family Responsibility Office has been criticized for years by Ontario's auditor general, who reported in 2010 that the agency was causing undue hardship for people who need the payments by not doing its job properly.
Auditor general Jim McCarter said the agency was struggling with the same problems his office raised in a 2003 report, and noted 80 per cent of phone calls to the office never get through because it is so overwhelmed.