The City of Hamilton says a pipe that had been leaking undetected for 26 years dumped an estimated 337 million litres of sewage into the local harbour.
Hamilton's mayor had called last week for an investigation after the leak was discovered, with preliminary findings suggesting a contractor put a hole in the pipe in 1996.
The city says it developed its estimate of sewage sent into the harbour based on water meter usage data for the properties connected to the leaking pipe.
It says the pipe was repaired last week, with the cost of fixing the leak estimated at $29,830.
The city says there was no danger to the drinking water supply of properties serviced by the pipe, and residents had been notified.
The city says it told the Environment Ministry about the amount of sewage believed to have been sent into the harbour over the last two and a half decades.
The city said last week that the leak was discovered by staff doing separate maintenance work.
It said it believed a contractor made the hole in the pipe under the impression that "all pipes were storm sewers'' designed to connect to culverts spilling into the harbour.