People are being told to stay out of the water at Melbourne's most popular beaches after heavy rain over the Christmas week washed large amounts of waste into the sea.
With high levels of faecal pollution around Port Philip Bay, swimmers are risk of catching gastroenteritis, says Dr Anthony Boxshall from the Environment Protection Authority (EPA).
There's contamination at 21 of the 36 beaches tested in Victoria in the past few days. The affected beaches span from Werribee South to Frankston.
The EPA looks in the water for a bacteria called enterococci. If there are 400 or more enterococci per 100 ml water, there's a 5 to 10 percent risk of sickness.
Children and elderly are most at risk of catching illnesses from contaminated water, Dr Boxshall says.
The water is expected to clear up by the end of the first week of January, providing there is no more rain.