The warnings affect parts of the Welsh coast including Barry Island, Penarth, and Pembrokeshire.
Discharges were reported early on August 31, after days of rain raised pollution risks.
According to Surfers Against Sewage, Jacksons Bay, Whitmore Bay, Watch House Bay, and Cold Knap in Barry were all affected by sewage discharges starting at 2.46am, with Welsh Water allegedly responsible for the discharges.
Penarth Beach was also impacted, with a discharge recorded at 12.59am on the same day.
Further west, Manorbier in Pembrokeshire was affected by an earlier discharge that began at 9pm on August 30, while Dale saw a discharge start at 6.16pm on August 31.
A pollution risk forecast has also been issued by Natural Resources Wales for Newport North on Tuesday, September 1.
Welsh Water and Natural Resources Wales have been approached for a comment.
