Sydney Harbour is in line for a "pollution injection" under a plan to funnel more sewage overflow into its waters during heavy storms, residents are being warned.

Plans are underway for more than a dozen new overflow pipes designed to release stormwater mixed with untreated waste water directly into Sydney Harbour and other NSW waterways during extreme rainfall.

The pipes are designed to stop wastewater flowing back into people’s homes at times when the city’s sewage pipes flood, on average twice per year.

Sydney Water says there are already 3000 in the network, and the overflow released is more than 99 per cent water.

However, environmentalists and the NSW opposition have slammed the plan as a dirty, cheap and old-fashioned fix, and called for greener alternatives.

Conservationist Jeff Angel from the Total Environment Centre said overflow water contained a hazardous mix of chemicals, microplastics and street runoff that created a “toxic plume” in Sydney’s waterways.

“This is not a clean, pure solution from Sydney Water,” he said.

“This is a pollution injection into the harbour.”

Locations slated for new overflow pipes include Mosman, Drummoyne and Putney, as well as Berkeley, Keiraville, Fairy Meadow and Shellharbour in the Illawarra region.

Greens councillor Pauline Tyrrell of Canada Bay Council, which includes Drummoyne, said the “disappointing” plan flies in the face of a local campaign to make the Parramatta River safe for swimming.

“Something like this would just ruin all the efforts to try to make things cleaner and ensure people don’t have to travel so far to swim,” Ms Tyrrell told AAP.

The NSW opposition blamed the government for “siphoning” profits from Sydney Water, quoting Auditor-General figures that showed it received $911 million in 2015, up 40 per cent from 2014.

Opposition Leader Luke Foley said the money should be invested in programs and infrastructure to capture and clean up sewage.
“This is just a wholesale reversal of three decades of improvement,” he said.

“This is 1950s stuff, just dumping raw sewage into Sydney Harbour.”

Sydney Water said it continued to make a significant investment in water infrastructure to ensure a reliable system.

It spent $489 million on infrastructure last year and this year planned to spend $517 million, a spokeswoman said.

“The allegation that we have diverted money away from infrastructure to dividends is simply untrue,” she said.