After torrential downpours on Sunday night, Victoria’s Environment Protection Authority (EPA) on Tuesday reiterated that all of Port Phillip Bay’s 36 beaches remained unsafe for swimming. Those ratings remain in place on Wednesday morning, on a day where the temperature is set to hit 34 degrees.

All the bay beaches received a “poor” rating by the EPA, which discourages contact with the water for between 24 and 48 hours. The heavy rainfall on Sunday increased bacterial levels in the bay, leading to concerns of gastro and infection for swimmers.

Although children and the elderly are at greater risk of contracting an illness, the EPA is urging all swimmers to steer clear.

Although Melbourne’s summer so far has been “pretty standard”, according to Dr Anthony Boxshall of the EPA, the storms Melbourne experienced in January and last Sunday “broke the mould a bit”.

Dr Boxshall said Port Phillip’s geography is part of the reason for its susceptibility to changes in water quality.

“Because of that little exit at the bottom end and the warmer fresh water at the top end – 60 percent of the fresh water that comes into the bay comes out the Yarra and the Maribrynong – water takes a longer time to leave the bay.”

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The EPA’s forecasting model for water quality relies on measurements of projected and actual rainfall, historical data, and the amount of light (as light kills bacteria). “We’ve had to relook at the model a bit after these really big events,” Boxshall said.

With Melbourne's temperature set to hit 34 degrees today, Dr Boxshall said that some beaches may see upgraded water quality, but those that are closer to the river outlets and the stormwater outlets will "probably the ones that take longer to recover.”

For updates on which beaches are and are not safe for swimming, visit Yarra Watch and follow Victoria EPA on Twitter.

This article was updated on February 8.