The largest disruption to Winnipeg's wastewater system in more than two years allowed more than three million litres of diluted sewage to be emptied into the Assiniboine River earlier this week.

Beginning Sunday night, an estimated 3.26 million litres of stormwater-diluted raw sewage poured into the city's waterways for three and a half hours, the City of Winnipeg reported on its website tracking the release of untreated sewage.

The discharge occurred at the Aubrey Wastewater Lift Station, located at 1016 Palmerston Avenue in Wolseley, and was due to a power outage.

It is the largest spill the city has recorded since five million litres of raw sewage leaked into the Red River in early 2016. Other releases may have been higher since but the city's website lists some incidents with unknown discharge amounts.

The city was alerted to the most recent leak Sunday night at 11:44 p.m. when a staff member at the McPhillips Control Centre received notice by alarm of a power failure at the Aubrey station, according to the city's website.

Standby crews rushed to the facility where they got a station pump operational by 1:10 a.m. on Monday. The station returned to normal operation shortly before 6 a.m. once Manitoba Hydro restored power.

The Aubrey station had two hours of stormwater storage capacity, but the power failure prevented the lift station from taking advantage of this capacity, city spokesperson Tamara Forlanski said.

"If a rainfall event had not occurred during the power outage, it is likely we would have been able to prevent untreated sewage from entering the river," she said in a statement.

Power failure at same site

The fourth worst discharge of the year happened on June 14 because of a power failure at the same lift station. The city's website says 2.4 million litres of sewage poured into the river in four hours.

Brian Mayes, chair of the city waste and water committee, said another discharge of untreated sewage is unsettling but unavoidable.

"We continue to have spills and we're going to continue to have some spills for decades to come, but we are working to improve the situation," he said. "Minister [Rochelle] Squires, one of the first things she said to me is, 'We've got to do something about this.'"

Last year, the province ordered the city to cut back on combined-sewer overflows from its antiquated system by 85 per cent by 2045. Winnipeg officials pegged the cost of replacing enough of these sewers, which can carry both stormwater runoff and raw sewage and overflow, at more than $1 billion.

"The work's going to continue, the work's going to accelerate," Mayes said. "This is one file where we seem to be working pretty well with the province, I'll just say that."