Authorities are warning of high levels of faecal pollution at some of Melbourne's most popular beaches after recent flash flooding, sparking concerns about gastroenteritis.
EPA Victoria is forecasting poor water quality - indicating unsuitable conditions for swimming - at 21 of the 36 beaches it monitors in Port Phillip Bay.
The affected beaches span from Werribee South on Melbourne's south-western fringe to Frankston, about 40 kilometres south-east of the Melbourne CBD.
Dr Anthony Boxshall, the EPA's manager of applied sciences, said last week's heavy storms and other recent rainfall had washed waste into the bay from the Yarra River.
"We have indicators we look for which is an indicator of faecal contamination, which is a nice way of saying poo," he told the ABC.
"It's bird poo, horse poo, cow poo and people poo, and that's everything that washes in from the streets coming through the storm system."
For its forecasts, the EPA samples water for a bacteria known as enterococci.
A poor quality forecast suggests levels of 400 or more enterococci per 100 millilitres of water, representing between 5 and 10 per cent risk of illness.
During dryer periods, levels are generally under 40 organisms per 100ml - representing a risk of illness of less than 1 per cent.
Dr Boxshall said the swimmers or other beachgoers who ingested contaminated water were at risk of getting gastroenteritis and other illnesses.
"What we know from international research from the World Health Organisation … is that if you swim in that kind of water with these levels and ingest the water, you can get sick," he said.
"It's mainly gastro … for some people it can get serious, and that's why we put out these warnings."
Children and the elderly were most at risk of falling ill from contaminated water.
Beaches on the Mornington Peninsula south of Frankston currently have a "good rating" and are still suitable for swimming.
The EPA forecasts for water quality are based on weather, 25 years of water quality history, recent bacterial sampling results and pollution reports.
Forecasts are available on the Yarra and Bay website and on signs at 28 Life Saving Victoria clubs around Port Phillip Bay.