He said in addition to the sewage, water from showers, baths, dishwashers and washing machines would also have flowed into the river.
“We knew we were on to something big as soon as we started sampling – the contamination was off the charts,” he said.
The team lifted a series of manholes to trace the issue and eventually arrived at Upper Grosvenor Road.
“We put our CCTV camera down the surface water pipe and couldn’t believe our eyes – rows of private sewage pipes were punched through the side and into the drainage pipe,” Mr Trinder recalled.
“This pipe then runs straight into the Itchen.”
Darko Zlatarek, the misconnections team leader, said the case was “on another level”.
He described the Itchen as “such a sensitive habitat” and said Southern Water was “spending millions in the area to protect wildlife and support a council application for a new bathing water designation”.
The company was hit with a £90m fine four years ago, after raw sewage was discharged across Hampshire, West Sussex and Kent.