Heavy storms wash faecal matter into Melbourne's Port Phillip Bay, prompting gastro risk warnings

Swimmers are being warned of gastro risk at Melbourne beaches because recent heavy storms have washed poo into Port Phillip Bay.

Environmental Protection Victoria's website has listed 21 of the 36 beaches it monitors around the bay as having poor water quality on Monday, meaning the water is not suitable for swimming.

The EPA's Anthony Boxshall says waste has washed into the bay following last week's heavy storms and the more recent, lighter rainfall.

"We have indicators we look for (in water tests), which is an indicator of faecal contamination, which is a really nice way of saying poo," Dr Boxshall told ABC News 24 on Monday.

"It's bird poo, it's horse poo, it's cow poo and it's people poo.

"It is everything that washes in from the streets, and everything that comes out through the storm water system."

Swimmers who ingest contaminated water have a higher risk of getting gastroenteritis and other illnesses, particularly the elderly and children.

Beaches on the Mornington Peninsula south of Frankston are still suitable for swimming.

Dr Boxshall says the water should clear up in 24-48 hours if it doesn't continue to rain.

More tests will be conducted this week and results posted on the EPA website.

The Bureau of Meteorology forecasts a maximum of 19 degrees on Monday, with light drizzle in the morning and evening.