The group found three dead coot chicks and another which was badly polluted but unable to be retrieved.
The group reported that Southern Water have had a team working to contain the pollution since Tuesday evening, and are using pollution mats, skimmers and booms to soak up the diesel.
Simon Wild, a spokesman for the rescue group said: “We have had fears for a swan which is sitting on eggs, but Southern Water are doing a fantastic job in trying to limit the damage, and we are hopeful that it will be alright.
"I expect that someone has poured no more than two or three gallons of diesel down a storm drain, but it has caused great damage and expense.
"I know Southern Water are looking to trace the source, and if they catch the person responsible there is going to be a huge fine.”