Melbourne’s stormwater drains were flooded with raw sewage for the third time in two months last night when parts of the city’s drainage system were overwhelmed by torrential rain, sparking outrage among environmentalists.

Melbourne Water today revealed that untreated effluent was pumped into Merri Creek at Coburg, Moonee Ponds Creek at Pascoe Vale and Gardiners Creek at Ashwood in a bid to keep the fouled water from flowing back into people’s homes.

The flooded underpass at Mount Waverley station. Credit:Gauthier Delmee

Last night’s release followed similar mass dumpings on February 5 and March 13, when Melbourne Water

pumped unknown quantities of sewage into the Yarra River, Maribyrnong River, Merri Creek, and Moonee Ponds Creek, with drains in Elwood, Brighton and Hampton also carrying fouled water into the bay.

A Melbourne Water spokesman said the sewage that could have threatened homes was "extremely diluted" because it was mixed with stormwater, and when it flowed into rivers it was further diluted.