Poor water quality has been forecast at 21 of the 36 beaches which the Environment Protection Authority (EPA) checks.

A “poor” quality assessment typically means an illness risk of between 5% and 10%, the BBC said, with the contaminated water potentially leading to gastroenteritis, especially in children and the elderly.

The beaches at risk are understood to run from Werribee South in Melbourne’s south-west, to Frankston which lies about 25 miles south of the city’s central business district.

Dr Anthony Boxshall, the EPA’s manager of applied sciences, told ABC News 24: “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."

"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."

He added that the water should be back to normal within around 24 to 48 hours, unless there is more rain.