Almost 56,000 cubic metres of sewage was dumped in a river in 2023, according to campaigners.
The Cleddau Project said Welsh Water data showed pumps at Picton in Haverfordwest, Pembrokeshire, were running at 80% capacity from July 2022 to February 2024.
That meant the equivalent of 17 tankers a day, which should have been going to a sewage treatment plant, was instead going into special conservation area the Western Cleddau.
Welsh Water apologised and said its Picton pumping station had been upgraded and spills cut.