So which beach is the best?

The Age has obtained 15 years of beach quality data from the Environment Protection Authority, and it shows there are eight beaches that have had an unbroken run of good years.

These are Blairgowrie, Hampton, Portarlington, Safety Beach, Sandridge, Sandringham, Sorrento and Eastern Beach, in Corio Bay. All eight have met their water quality targets every year since the summer of 2001/02.

Aspendale, Black Rock, Brighton, Carrum, Mornington, Mount Martha, Rosebud, Santa Casa and South Melbourne have also displayed great water quality.  (However there have only been tests at these locations for the past two years.)

That means that 75 per cent of the time, the levels of gastro-linked bacteria at these spots have been well within safe levels.

But what about beaches that have have been teeming with germs?

Mentone Beach is the standout. In six of the past 15 years it has not met water quality targets.

Last year it was the only beach in the bay not to make the grade after five of the 14 weekly summer water tests found bacteria concentrations above the safe level of 200 organisms in every 200 ml of water. On one bad week, it recorded bacteria levels almost five times the safe amount.

St Kilda Beach and Elwood Beach did not make the grade five times, while Frankston and Altona beaches failed four times.

But that isn't to say that you're risking an explosive case of gastro every time you dip your toe into the waters at these beaches. Most of the time the water is fine, but they can be badly hit after heavy rain.

And after heavy rains, the EPA will typically advise Melburnians to avoid swimming at all Port Phillip Bay beaches for about 48 hours, by which time the bacteria washed in by stormwater will have disintegrated.

So far this summer, The Dell, Portarlington, St Leonards and Eastern Beach have had the most consistent good forecasts.

The water quality forecast for Saturday was looking good for all 36 monitored bay beaches except those at Frankston, St Kilda and Port Melbourne.

The EPA's Anthony Boxshall says several factors can affect a beach's water quality:

• Outflows from the Yarra River: The currents from the bay tend to move in a clockwise direction, so once water from the Yarra flows into the bay it gets pushed down Port Phillip Bay's eastern flank, past St Kilda, Elwood and towards Frankston. And even though they are the two beaches closest to the mouth of the Yarra, Sandridge and South Melbourne are protected from the river's outflows by the bay's currents.
• Stormwater drains and creeks: St Kilda, Altona, Elwood, Frankston and Mentone beaches are all close to creeks or stormwater drains. Whenever it rains, faecal matter from animals is invariably in the mix. It's for this reason that beaches that are quite close together can record vastly different results. Elwood Beach has had five bad years, but if you walk about five kilometres down the coast to Hampton Beach you reach an area with an unbroken run of good years - that's because Elwood canal empties out into the area around Elwood beach. 
• Proximity to the bay's entrance. Beaches close to the entrance of the bay like Portsea, Sorrento and Queenscliff, all the way up to the line from St Leonards to Canadian Bay, have their waters recycled more often thanks to flows in and out of Bass Strait. However, further north the same water can linger for about a year.

Dr Boxshall said weekly data on bacteria levels was one piece of information the EPA used to compile its twice-daily beach forecasts during the summer months, along with any recent rainfall data and weather forecasts.

If rain is on the way, a beach's prognosis may change. A cloudy day is also a bad sign since the lack of sun allows bacteria to thrive. He said the EPA's forecasts of good water quality at a given beach were correct about 90 per cent of the time. .

But the formulas cannot predict one-off events, so if there are any reports of sewage overflows, pollution spills, leaky portaloos or some dodgy plumbing pumping waste into stormwater, the EPA will adjust a beach's forecast accordingly.

Here's the full map of the Bay's best and worst beaches: