There seems to be a simple rule of thumb for finding a clean beach in Melbourne: the further it is from the mouth of the Yarra, the better its water quality.
The twice-daily beach reports for this summer so far - and pretty much every summer - show that locations towards Geelong and the entrance to Port Phillip Bay are more pristine than areas closer to the city.
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Melbourne's best and worst beaches to swim
Beaches out west are best, but at some of the most popular spots near the city, there's something in the water and it's not pretty.
On 15 days since the start of November, the Environmental Protection Authority's forecasts have warned Melburnians to think twice about going for a swim at St Kilda, Port Melbourne or Elwood beaches.
These three areas are notorious for poor water quality because of their proximity to the Yarra and large stormwater drains, and recorded levels of gastro-linked bacteria of up to 10 times the acceptable limit in the lead-up to Christmas.
The prognosis has also been poor at Canadian Bay and Dromana beaches on the Mornington Peninsula.
Even though these areas are far from the mouth of the Yarra, pollution risk from nearby Ballar Creek led the EPA to issue a "poor" forecast at Canadian Bay on November 19.
And a high bacteria reading at Dromana beach during dry weather - which the authority believes could have come from a business or waste truck dumping sewage in a stormwater drain - led to a poor forecast on December 10.
On both of these days, the all-clear was given for swimming at the rest of the bay's 36 monitored beaches.
But Canadian Bay locals were quick to defend their beach on Wednesday, saying nearby Frankston beach was much dirtier.
"This is a lovely beach; it's very secluded and private," Chris Storen said. "It's never that busy and has the cliffs around it, which block out the wind."
Mr Storen told The Age that he expected to "live to tell the tale" whenever he went for a swim at Canadian Bay.
But at any of these beaches so far this summer, there has been about a one in four chance that taking a dip would have been a bad idea.
On the other hand, over at Melbourne's most pristine seaside spots there has been about a one in 10 chance of murky water.
For the second year, the best beach in Melbourne has been Eastern Beach in Geelong, which has had 54 out of 61 days of perfect waters.
The popular family holiday spot – which oddly enough is the westernmost beach the EPA monitors – features an enclosed swimming area, diving boards and a children's pool.
Nearby beaches on the Bellarine Peninsula, such as Portarlington, St Leonards and the Dell, have also fared well this summer, along with the holiday hotspots a short ferry ride away in Sorrento, Portsea and Blairgowrie.
EPA applied sciences group manager Anthony Boxshall said all beaches in Port Phillip Bay had performed well over the summer because there had not been much rain.
Pollution accumulates during dry spells and is flushed into the bay during a downpour, so the authority advises people to avoid swimming near stormwater or river outlets after wet weather.
Heavy rains on Boxing Day meant all 36 beaches were deemed unsuitable for swimming for two days afterwards.
The EPA beach reports use a mixture of weather forecasts, pollution reports and historical water data to predict water quality.
But the forecasts cannot account for beaches being affected in other ways. While Black Rock has had good water forecasts, midnight ravers trashed its foreshore last month, leaving behind a trail of rubbish and empty bottles.
St Kilda residents also awoke to find six tonnes of rubbish from a rave strewn on their beach on Boxing Day, which led Port Phillip council to ask New Year's revellers not to do the same.
Dr Boxshall said the authority urged Melburnians to report suspected algal blooms, pollution or fish deaths during the summer.
EPA Pollution Hotline: 1300 372 842
Melbourne's top five beaches for water quality so far this summer
- Eastern Beach
- The Dell
- Santa Casa
Melbourne's worst five beaches for water quality so far this summer
- St Kilda
- Port Melbourne
- Canadian Bay