Hayling Sewage Watch said that five sewage outfalls were discharged into Langstone Harbour over a 61.5-hour period last weekend.
During this period, no warning notices were posted on Hayling Island’s Blue Flag-flying beach.
However, Havant Borough Council has rejected the group’s claim that the discharged water is dangerous, saying that it was ‘storm wastewater’.
On June 16, only 24 hours before the water discharged into the harbour, the council held a full meeting to discuss a petition with more than 2,000 signatures from people calling for electronic beach signage providing users with real-time water pollution information.
The council said that electronic signs would not be practical for the seafront.
Mike Owens of Hayling Sewage Watch said: ‘Back in March, Havant borough councillors, and others, published a letter they wrote to the minister for the environment calling for immediate action on harbour pollution.
‘But when they had the opportunity to act to protect residents and visitors they missed it.
‘Ironically while the pollution was pouring into Langstone Harbour the council was publicly boasting about Hayling keeping its Blue Flag beach after a Keep Britain Tidy visit. You really couldn’t make it up.
‘The solids are removed from the polluted water that is released into the harbours.
‘It is the stuff you can’t see that is most dangerous - the revolting cocktail of bacteria, viruses, chemicals, pharmaceuticals and metals that can make you ill and damages the environment.
‘At least we have the choice to enter the water or not. Fish and fauna do not.’
Cllr Clare Satchwell, cabinet lead for planning, Hayling seafront strategy and coastal management, said: ‘We have received notification from the Environment Agency, which monitors discharges by Southern Water, that all recent discharges have been compliant, and that this is not unusual during periods of heavy rainfall.
‘Discharges of this nature are storm wastewater, not untreated raw or unscreened sewage.
‘Weekly sampling undertaken by the Environment Agency during the bathing season consistently returns excellent water quality status for the area.
‘However, residents are encouraged to check Southern Water’s Beachbouy app following periods of heavy rainfall.’