3 Of Melbourne's beaches have been closed due to bacteria from faeces. Photo: Justin McManus

Sydney - Australian officials have warned of the risk of contracting diarrhea at popular Melbourne beaches after they were deemed unsafe for swimming due to high levels of faecal bacteria following storms over the weekend.

The Environment Protection Authority forecast "poor" water quality at eight beaches at Port Phillip Bay and warned people to avoid them due to the risk of contracting gastroenteritis, also known as diarrhea.

A "poor" quality forecast carries an illness risk of five to ten per cent.

Torrential storms have been blamed for washing faeces into the water.

"This is an unusual event. In Melbourne, we had an enormously large thunderstorm and rainfall this weekend - one of those one in a hundred years storm," Anthony Boxshall, the group manager of applied sciences at the authority, told dpa on Thursday.

"When there is that much amount of water it picks up everything from the street and parks and everywhere to the bay. Everything on the land ends up at the ocean and from all kinds of sources."

"So there is dog poo, duck poo, cow poo, bird poo and people poo."

Boxshall said people's faeces could have come from leaky portable toilets, as well as spills and leaks from the sewerage system.

"But all poo are bad and equally icky. They all have pathogens that are harmful and this happens everywhere, in every big city."

Heavy rainfall predicted for Thursday could further contaminate the waters, Boxshall said, since "rain is not the friend of beach water quality."

There were no reports of any outbreaks of health-related issues as of Thursday in the city of four million people.

There are more than three dozen beaches in the Melbourne area. Though not as famous as Sydney's beaches, visitors like Melbourne for its calm, shallow and flat sandy beaches close to the city.