Cockle pickers in the Burry Estuary said the leak follows a summer "wipeout" which had already destroyed thousands of cockles.
They claim it is the latest incident in a water pollution issue which they say is being ignored by the authorities.
Natural Resources Wales said it was investigating and that samples had been taken for analysis.
Welsh Water said teams were working hard to maintain services and measure any environmental impact. It said there had been no reported change to the status of the shellfishery beds.
The latest incident involved a break in an outlet pipe near Gowerton sewage works which engineers have been working to repair.
A sewage spillage has contaminated cockle beds in south Wales, it has been claimed.